glx: fix null pointer deref in __glXGenerateError()

glx: fix null pointer deref in __glXGenerateError()

The gc var would be NULL if called from line 238.  Instead, get
the opcode from __glXSetupForCommand(dpy) as done in other places.
And remove the unused gc parameter.

Fixes a bug reported by "John Doe" on 3/9/2011.

NOTE: This is a candidate for the 7.10 branch.