Check if dest is src before strncpy in Q_strncpyz.

I stumbled into this after updating MacOSX to 10.9 (Mavericks). IOQuake3, XreaL and ETLegacy where crashing after the update. This commit checks if we try to copy a string with same src and dest. More info here:
http://dev.etlegacy.com/issues/385
GDB output here (line 15):
http://pastebin.com/qGm4WvMM