lib: added check to prevent int overflow

If enough large input is given ofpact_finish will fail.
Implemented ofpbuf_oversized function to check for oversized
buffer. Checks were added for parse functions and error messages
returned.

Basic manual testing performed.

Reviewed-by: Yifeng Sun <pkusunyifeng@gmail.com>
Reported-by: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=12972
Signed-off-by: Toms Atteka <cpp.code.lv@gmail.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>

lib/learn.c

@@ -455,6 +455,11 @@ learn_parse__(char *orig, char *arg, const struct ofputil_port_map *port_map,
             learn = ofpacts->header;
         }
     }
+
+    if (ofpbuf_oversized(ofpacts)) {
+        return xasprintf("input too big");
+    }
+
     ofpact_finish_LEARN(ofpacts, &learn);
 
     return NULL;