Specify return behavior in header for evbuffer_pullup() in corner case

Function returns NULL when told to pullup more data than exists

include/event2/buffer.h

@@ -944,7 +944,8 @@ void evbuffer_cb_unsuspend(struct evbuffer *buffer, struct evbuffer_cb_entry *cb
   @param buf the evbuffer to make contiguous
   @param size the number of bytes to make contiguous, or -1 to make the
 	entire buffer contiguous.
-  @return a pointer to the contiguous memory array
+  @return a pointer to the contiguous memory array, or NULL if param size
+	requested more data than is present in the buffer.
 */
 
 unsigned char *evbuffer_pullup(struct evbuffer *buf, ev_ssize_t size);