gallium: move assertion (though not really significant)

src/mesa/state_tracker/st_draw.c

          pipe_reference_buffer(pipe, &vbuffer[attr].buffer, stobj->buffer);
          vbuffer[attr].buffer_offset = (unsigned) arrays[mesaAttr]->Ptr;
          velements[attr].src_offset = 0;
-         assert(velements[attr].src_offset <= 2048); /* 11-bit field */
       }
       else {
          /* attribute data is in user-space memory, not a VBO */
          velements[attr].src_offset = 0;
       }
 
+      assert(velements[attr].src_offset <= 2048); /* 11-bit field */
+
       /* common-case setup */
       vbuffer[attr].pitch = arrays[mesaAttr]->StrideB; /* in bytes */
       vbuffer[attr].max_index = max_index;