ofp-util: Reload ofpbuf pointers after nx_put_match().

ofp-util: Reload ofpbuf pointers after nx_put_match().

Fixes a wild pointer write when nx_put_match() expands the ofpbuf.

The change to the OFPST_FLOW case isn't strictly a bug fix, it just makes
the code for that case more closely resemble the NXST_FLOW case.

Bug #12403.
Bug #12460.
Bug #12461.
Bug #12481.
Reported-by: Timothy Chen <tchen@nicira.com>
Reported-by: Natasha Gude <natasha@nicira.com>
Reported-by: James Schmidt <jschmidt@nicira.com>
Reported-by: Alan Shieh <ashieh@nicira.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>