mesa: Fix error checking for getters consisting of only API versions.

mesa: Fix error checking for getters consisting of only API versions.

In almost all of our cases, getters that are turned on for only some API
variants will have an extension listed as one of the things that can
enable it, and thus api_check gets set.  For extra_gl30_es3 (used for
NUM_EXTENSIONS, MAJOR_VERSION, MINOR_VERSION) on a GL 2.1 context, though,
we would check twice, not find either one, but never actually throw the
error.