Fixing small memory leak in Cocoa

This only showed up when I set up a test harness doing several hundred setup/teardowns of my engine (which involved equally many calls to glfwInit and glfwTerminate). It's a pretty minor thing, obviously, and not likely to be noticed since I imagine most apps don't do multiple invocations like that, but every little bit helps. :) 

I'm also seeing a memory leak from the calloc calls in _glfwPlatformGetMonitors, but I haven't been able to track that down as it looks from my end like all that memory is appropriately freed in _glfwDestroyMonitor(s).