account for head space in cmdu_realloc()

src/cmdu.c

@@ -336,7 +336,7 @@ struct cmdu_buff *cmdu_realloc(struct cmdu_buff *c, size_t size)
 	if (size < origsize)
 		return c;
 
-	n = realloc((uint8_t *)c, size);
+	n = realloc((uint8_t *)c, sizeof(*f) + size + CMDU_RESERVE_HEADSPACE);
 	if (!n)
 		return NULL;