ofproto: Fix use-after-free in bridge destruction with groups.

ofproto: Fix use-after-free in bridge destruction with groups.

Groups were not destroyed until after lots of other important bridge
data had been destroyed, including the connection manager.  There was an
indirect dependency on the connection manager for bridge destruction
because destroying a group also destroys all of the flows that reference
the group, which in turn causes the ofmonitor to be invoked to report that
the flows had been destroyed.  This commit fixes the problem by destroying
groups earlier.

The problem can be observed by reverting the code changes in this commit
then running "make check-valgrind" with the test that this commit
introduces.

Reported-by: Simon Horman <simon.horman@netronome.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>
Reviewed-by: Simon Horman <simon.horman@netronome.com>