mesa/main: remove bogus error for zero-sized images

The explanation quotes the spec on the following wording to justify the
error:

"An INVALID_VALUE error is generated if xoffset + width is greater than
 the texture’s width, yoffset + height is greater than the  texture’s
 height, or zoffset + depth is greater than the texture’s depth."

However, this shouldn't generate an error in the case where *all three*
of width, xoffset and the texture's width are zero. In this case, we end
up generating an unspecified error.

So let's remove this check, and instead make sure that we consider this
as an empty texture.

So let's not generate an error, there's non mandated in the spec in
xoffset/yoffset/zoffset = 0 case. We already avoid doing any work in
this case, because of the final, non-error generating check in this
function.

Fixes: b37b35a5d26 "getteximage: assume texture image is empty for non defined levels"
Signed-off-by: Erik Faye-Lund <erik.faye-lund@collabora.com>
Reviewed-by: Juan A. Suarez <jasuarez@igalia.com>