Fix various warnings: [-Waddress], [-Wpointer-arith], [-Wwrite-strings], [-Wreturn-local-addr] and more

Fixes the following GCC 5 warnings:
```
core/os/file_access.cpp:49:19: warning: the address of 'FileAccess::create_func' will always evaluate as 'true' [-Waddress]
servers/audio_server.cpp:192:70: warning: comparison with string literal results in unspecified behaviour [-Waddress]

drivers/gles2/rasterizer_storage_gles2.cpp:4095:90: warning: NULL used in arithmetic [-Wpointer-arith]

modules/gdnative/register_types.cpp:237:3: warning: deprecated conversion from string constant to 'char*' [-Wwrite-strings]
platform/android/export/export.cpp:207:1: warning: deprecated conversion from string constant to 'char*' [-Wwrite-strings]

modules/gdscript/gdscript.h:150:67: warning: returning reference to temporary [-Wreturn-local-addr]
servers/physics_2d/collision_object_2d_sw.h:119:56: warning: returning reference to temporary [-Wreturn-local-addr]
servers/physics_2d/collision_object_2d_sw.h:123:56: warning: returning reference to temporary [-Wreturn-local-addr]
servers/physics_2d/collision_object_2d_sw.h:127:50: warning: returning reference to temporary [-Wreturn-local-addr]
servers/physics_2d/collision_object_2d_sw.h:131:52: warning: returning reference to temporary [-Wreturn-local-addr]

editor/plugins/skeleton_editor_plugin.cpp:34:36: warning: extra tokens at end of #include directive
modules/bullet/bullet_types_converter.cpp:31:9: warning: #pragma once in main file

editor/import/editor_scene_importer_gltf.cpp:1996:51: warning: name lookup of 'i' changed
modules/visual_script/visual_script_property_selector.cpp:402:45: warning: name lookup of 'E' changed
scene/gui/tree.cpp:1268:25: warning: name lookup of 'i' changed
scene/resources/visual_shader.cpp:808:32: warning: name lookup of 'i' changed
```

4 files changed, 7 insertions, 9 deletions

diff --git a/modules/bullet/bullet_types_converter.cpp b/modules/bullet/bullet_types_converter.cpp
index a0fe598227..f9b7126173 100644
--- a/modules/bullet/bullet_types_converter.cpp
+++ b/modules/bullet/bullet_types_converter.cpp
@@ -28,8 +28,6 @@
 /* SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.                */
 /*************************************************************************/
 
-#pragma once
-
 #include "bullet_types_converter.h"
 
 /**
diff --git a/modules/gdnative/register_types.cpp b/modules/gdnative/register_types.cpp
index f56778b296..62e87c3651 100644
--- a/modules/gdnative/register_types.cpp
+++ b/modules/gdnative/register_types.cpp
@@ -29,19 +29,19 @@
 /*************************************************************************/
 
 #include "register_types.h"
+
 #include "gdnative/gdnative.h"
 
 #include "gdnative.h"
 
-#include "core/io/resource_loader.h"
-#include "core/io/resource_saver.h"
-
 #include "arvr/register_types.h"
 #include "nativescript/register_types.h"
 #include "net/register_types.h"
 #include "pluginscript/register_types.h"
 
 #include "core/engine.h"
+#include "core/io/resource_loader.h"
+#include "core/io/resource_saver.h"
 #include "core/os/os.h"
 #include "core/project_settings.h"
 
@@ -148,7 +148,7 @@ protected:
 };
 
 struct LibrarySymbol {
-	char *name;
+	const char *name;
 	bool is_required;
 };
 
diff --git a/modules/gdscript/gdscript.h b/modules/gdscript/gdscript.h
index d795500265..4a74b0712a 100644
--- a/modules/gdscript/gdscript.h
+++ b/modules/gdscript/gdscript.h
@@ -147,7 +147,7 @@ public:
 	const Map<StringName, Variant> &get_constants() const { return constants; }
 	const Set<StringName> &get_members() const { return members; }
 	const GDScriptDataType &get_member_type(const StringName &p_member) const {
-		ERR_FAIL_COND_V(!member_indices.has(p_member), GDScriptDataType());
+		CRASH_COND(!member_indices.has(p_member));
 		return member_indices[p_member].data_type;
 	}
 	const Map<StringName, GDScriptFunction *> &get_member_functions() const { return member_functions; }
diff --git a/modules/visual_script/visual_script_property_selector.cpp b/modules/visual_script/visual_script_property_selector.cpp
index 64a6c19f91..e2e5cc77f5 100644
--- a/modules/visual_script/visual_script_property_selector.cpp
+++ b/modules/visual_script/visual_script_property_selector.cpp
@@ -386,8 +386,8 @@ void VisualScriptPropertySelector::get_visual_node_names(const String &root_filt
 		}
 		Vector<String> path = E->get().split("/");
 		bool is_filter = false;
-		for (Set<String>::Element *E = filter.front(); E; E = E->next()) {
-			if (path.size() >= 2 && path[1].findn(E->get()) != -1) {
+		for (Set<String>::Element *F = filter.front(); F; F = F->next()) {
+			if (path.size() >= 2 && path[1].findn(F->get()) != -1) {
 				is_filter = true;
 				break;
 			}