mesa: fix error handling for dlist image unpacking

mesa: fix error handling for dlist image unpacking

When compiling glDrawPixels, glTexImage(), etc. and we're copying
the user's image we need to be careful about GL error checking.
Previously, we were incorrectly generating GL_OUT_OF_MEMORY in
unpack_image() if width <= 0 or height <= 0 or for invalid format/type
values.  We now check those arguments in unpack_image() and return NULL
if there's a bad value.  The command will get compiled with the
arguments as-is and image=NULL.  Later, when the command is executed the
correct errors will be generated.

This issue was reported by Yuanhan Liu <yuanhan.liu@linux.intel.com>

Reviewed-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>