mesa/pbo: Handle zero width, height or depth when validating access

mesa/pbo: Handle zero width, height or depth when validating access

It's legal to call glTexSubImage with zero values for the width,
height or depth. Previously this was breaking the PBO access
validation because it tries to work out the last pixel accessed by
getting the pixel at height-1 and depth-1 which would end up with
bogus values.

This was causing GL errors to be generated during the Piglit
texsubimage test, although the test was passing anyway.

v2: Also check for width == 0. Don't validate the start pointer if any
    of the dimensions are zero.
Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>