glamor: avoid a crash if texture allocation failed

Texture creation in _glamor_create_tex() can fail if a GL_OUT_OF_MEMORY
is raised, in which case the texture returned is zero.

But the texture value is not checked in glamor_create_fbo() and glamor
will abort in glamor_pixmap_ensure_fb() because the fbo->tex is 0:

  Truncated backtrace:
  Thread no. 1 (10 frames)
   #4 glamor_pixmap_ensure_fb at glamor_fbo.c:57
   #5 glamor_create_fbo_from_tex at glamor_fbo.c:112
   #6 glamor_create_fbo at glamor_fbo.c:159
   #7 glamor_create_fbo_array at glamor_fbo.c:210
   #8 glamor_create_pixmap at glamor.c:226
   #9 compNewPixmap at compalloc.c:536
   #10 compAllocPixmap at compalloc.c:605
   #11 compCheckRedirect at compwindow.c:167
   #12 compRealizeWindow at compwindow.c:267
   #13 RealizeTree at window.c:2617

Check the value returned by _glamor_create_tex() in glamor_create_fbo()
and return NULL in the texture is zero.

All callers of glamor_create_fbo() actually check the returned value and
will use a fallback code path if it's NULL.

Please cherry-pick this to active stable branches.

Bugzilla: https://bugzilla.redhat.com/1433305


Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
(cherry picked from commit 8805a48e)