Misc: Code clean-up

- Fix a memory leak in register_cm().

- Print a warning message if argument of --vsync is invalid.

- Known bug: compton will probably freeze X if another compositing
  window manager is running and --vsync opengl is enabled, with
  nvidia-drivers-304.51. Probably an issue on the driver. I could see no
  workaround.

src/compton.h

@@ -31,7 +31,6 @@
 // Whether to enable OpenGL VSync support
 // #define CONFIG_VSYNC_OPENGL 1
 
-#define NDEBUG 1
 // === Includes ===
 
 // For some special functions