Fix warnings on release builds (not DEBUG_ENABLED)

Fixes the following Clang 5 warnings:
```
modules/bmp/image_loader_bmp.cpp:46:60: warning: comparison of unsigned expression < 0 is always false [-Wtautological-compare]
modules/bmp/image_loader_bmp.cpp:48:61: warning: comparison of unsigned expression < 0 is always false [-Wtautological-compare]
drivers/png/image_loader_png.cpp:231:20: warning: comparison of unsigned expression >= 0 is always true [-Wtautological-compare]
scene/gui/graph_edit.cpp:1045:8: warning: comparison of constant 0 with expression of type 'bool' is always false [-Wtautological-constant-out-of-range-compare]

core/class_db.cpp:812:13: warning: unused variable 'check' [-Wunused-variable]
core/io/file_access_pack.cpp:172:11: warning: unused variable 'ver_rev' [-Wunused-variable]
core/math/bsp_tree.cpp:195:13: warning: unused variable 'plane' [-Wunused-variable]
core/math/bsp_tree.cpp:168:6: warning: unused variable 'plane_count' [-Wunused-variable]
modules/gdscript/gdscript_function.cpp:685:10: warning: unused variable 'ok' [-Wunused-variable]
modules/gdscript/gdscript_function.cpp:706:10: warning: unused variable 'ok' [-Wunused-variable]
modules/gdscript/gdscript_function.cpp:755:19: warning: unused variable 'var_type' [-Wunused-variable]
modules/gdscript/gdscript_function.cpp:1306:12: warning: unused variable 'err' [-Wunused-variable]

modules/gdscript/gdscript_function.cpp:158:15: warning: unused function '_get_var_type' [-Wunused-function]
modules/gdscript/gdscript_parser.cpp:750:20: warning: unused variable 'lv' [-Wunused-variable]
modules/gdscript/gdscript_parser.cpp:59:15: warning: unused function '_find_function_name' [-Wunused-function]
scene/main/node.cpp:2489:13: warning: unused function '_Node_debug_sn' [-Wunused-function]
```

0 files changed, 0 insertions, 0 deletions