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 reported from the `calloc` calls in `_glfwPlatformGetMonitors`, but I haven't been able to track that down since as far as I can tell all that memory is appropriately freed in `_glfwDestroyMonitor(s)`.