st: assert on pipe_buffer_create failure

This needs a proper fix to propogate the out-of-memory condition back
up to Mesa and the app as a GL error.  Until then, at least catch the
problem at its source.

src/mesa/state_tracker/st_cb_bufferobjects.c

@@ -169,6 +169,11 @@ st_bufferobj_data(GLcontext *ctx,
 
    st_obj->buffer = pipe_buffer_create( pipe->screen, 32, buffer_usage, size );
 
+   /* We don't seem to have any good way of passing failure to
+    * allocate up to Mesa??
+    */
+   assert(st_obj->buffer);
+
    st_obj->size = size;
 
    if (data)