Perform range checking on app supplied texture base level

Perform range checking on app supplied texture base level

It is possible for applications to specify any texture base level,
including trivially invalid values (i.e., 47000000).  When an app
specifies an invalide base level, we should gracefully disable the
texture instead of accessing memory outside the gl_texture_object.

This fixes an occasional segfault in one of our conformance tests.