mesa: fix compressed_subtexture_error_check() return value

mesa: fix compressed_subtexture_error_check() return value

The function should return GLboolean, not GLenum.
If we detect invalid compressed pixel storage parameters, we should
return GL_TRUE, not GL_FALSE so that the function is no-op'd.
An update to the piglit s3tc-errors test will check this.

Reviewed-by: Matt Turner <mattst88@gmail.com>