Fix some OSX and iOS Clang warnings
Fixes the following XCode 9.4.1 warnings:
```
core/os/memory.cpp:175:13: warning: unused variable 's' [-Wunused-variable]
drivers/coremidi/core_midi.cpp:68:14: warning: comparison between NULL and non-pointer ('MIDIEndpointRef' (aka 'unsigned int') and NULL) [-Wnull-arithmetic]
drivers/gles2/rasterizer_gles2.cpp:77:24: warning: unused function '_gl_debug_print' [-Wunused-function,34]
drivers/unix/thread_posix.cpp:106:12: warning: unused variable 'running_thread' [-Wunused-variable,34]
modules/gdnative/nativescript/nativescript.h:371:16: warning: in-class initialization of non-static data member is a C++11 extension [-Wc++11-extensions]
platform/iphone/gl_view.mm:56:14: warning: unused variable 'video_previous_volume' [-Wunused-variable,34]
platform/iphone/gl_view.mm:251:12: warning: unused function 'get_first_id' [-Wunused-function,34]
platform/iphone/main.m:45:15: warning: unused variable 'app' [-Wunused-variable,34]
platform/osx/os_osx.mm:79:15: warning: unused function 'convertRectToBacking' [-Wunused-function]
```
97b9697ea2