ofpbuf: Reintroduce assertion on size in ofpbuf_pull().

This assertion was removed as part of a commit that was intended to
just be a cleanup.

Fixes: 6fd6ed7 ("ofpbuf: Simplify ofpbuf API.")
Acked-by: Justin Pettit <jpettit@ovn.org>
Signed-off-by: Ben Pfaff <blp@ovn.org>

include/openvswitch/ofpbuf.h

@@ -246,6 +246,7 @@ static inline void ofpbuf_clear(struct ofpbuf *b)
  * 'size' bytes of data.  Returns the first byte of data removed. */
 static inline void *ofpbuf_pull(struct ofpbuf *b, size_t size)
 {
+    ovs_assert(b->size >= size);
     void *data = b->data;
     b->data = (char*)b->data + size;
     b->size = b->size - size;