i965: Drop assertion about buffer offset at draw time.

i965: Drop assertion about buffer offset at draw time.

Given robust access, we should just be returning zeroes if the user gives
us a base pointer that's too big, which is what was happens on a release
build.  This was caught by a webgl conformance test for out-of-bounds
draws on servo.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>