Fix USB endpoint read with no data (thanks nox771)
http://forum.pjrc.com/threads/24726-Teensy-3-0-Serial-available%28%29-bug?p=42926&viewfull=1#post42926
Merge branch 'master' of https://github.com/PaulStoffregen/cores

Conflicts:
	teensy3/usb_dev.c