mesa/fbo: do not assign a value that is never read later on

mesa/fbo: do not assign a value that is never read later on

The issue has been detected by coverty.

v2:
- move the declaration of obj to the else clause (Brian Paul)

v3: Review by Brian Paul
- get rid of the obj declaration in favor of a direct reference

Reviewed-by: Brian Paul <brianp@vmware.com>
Signed-off-by: Martin Peres <martin.peres@linux.intel.com>