vf: fix NULL pointer issue

"mpi" was accessed right after the if block, that checked that mpi is
not NULL. Either the check is uneeded, or the access to "mpi" always
crashes. Just move the access inside the checked block.

libmpcodecs/vf.c

@@ -358,8 +358,8 @@ mp_image_t *vf_get_image(vf_instance_t *vf, unsigned int outfmt,
             mpi->flags |= MP_IMGFLAG_TYPE_DISPLAYED;
         }
         mpi->qscale = NULL;
+        mpi->usage_count++;
     }
-    mpi->usage_count++;
     return mpi;
 }