mesa/main: factor out common error-checking

mesa/main: factor out common error-checking

This error checking is the same for teximage and texsubimage getters, so
let's factor it out to its own function.

This will be useful when getteximage and gettexsubimage gets their own
error checking routines a bit later.

Signed-off-by: Erik Faye-Lund <erik.faye-lund@collabora.com>
Reviewed-by: Juan A. Suarez <jasuarez@igalia.com>