The edgeflag comes in as ubyte with glEdgeFlagPointer but as float with
plain immediate glEdgeFlag. Avoid reading bytes that weren't meant for
the edgeflag in the pointer case.

Fixes intermittent failures with gl-2.0-edgeflag piglit (and valgrind
complaints about reading uninitialized memory).

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Cc: mesa-stable@lists.freedesktop.org