mesa: Check for a negative "size" parameter in glCopyBufferSubData().

mesa: Check for a negative "size" parameter in glCopyBufferSubData().

From the GL_ARB_copy_buffer spec:
"An INVALID_VALUE error is generated if any of readoffset, writeoffset,
 or size are negative [...]"

Fixes oglconform's copybuffer/negative.CNNegativeValues test.

NOTE: This is a candidate for stable release branches.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>