ofproto-dpif-upcall: Avoid use-after-free in revalidate() corner cases.

The loop in revalidate() needs to ensure that any data obtained from
dpif_flow_dump_next() is used before it is destroyed, as indicated by
dpif_flow_dump_next_may_destroy_keys().  In the common case, where
processing reaches the end of the main "while" loop, it does this, but
in two corner cases the code in the loop execute "continue;", which skipped
the check.  This commit fixes the problem.

Bug #1249988.
Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Joe Stringer <joestringer@nicira.com>