IPReassembler: pack ChunkLink, since it can be arbitrarily placed in …

…Packet::data

These two uint16_ts may end up placed at arbitrary offsets into a packet.
In order to generate proper unaligned read/write instructions, mark the
struct as packed.

This was exposed by Defensic suite IPv4 Header Fragment-offset, test 45067
and discovered by Waldin Stone.  I verified it by asserting proper alignment
of ChunkLinks and found that they were not always properly aligned.

elements/ip/ipreassembler.hh

@@ -84,7 +84,7 @@ class IPReassembler : public Element { public:
     struct ChunkLink {
 	uint16_t off;
 	uint16_t lastoff;
-    };
+    } __attribute__((packed));
 
   private: