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.

Makefile

@@ -38,6 +38,7 @@ ifeq "$(NO_VSYNC_OPENGL)" ""
 	LIBS += -lGL
 endif
 
+CFLAGS ?= -DNDEBUG
 CFLAGS += $(CFG)
 
 LIBS += $(shell pkg-config --libs $(PACKAGES))