ofproto: Fix bad memory access sending large numbers of port stats replies.

ofproto: Fix bad memory access sending large numbers of port stats replies.

The append_stats_reply() function can modify its pointer argument, but
append_port_stat() was failing to propagate this change back to its own
caller.  So when append_stats_reply() did in fact modify it (which happens
when the 64 kB maximum OpenFlow message length was exceeded), the
handle_port_stats_request() function would then access freed memory on the
next call to append_port_stat().

Bug #2714.
Reported-by: Ram Jothikumar <rjothikumar@nicira.com>
Debugging help by Justin Pettit <jpettit@nicira.com>