rconn: Fix null pointer dereference in rconn_add_monitor().

rconn_add_monitor() tries to check the version of the controller
connection being monitored, so that it can decide what OpenFlow version to
tell the monitor connection to negotiate.  But at any given time an rconn
may not have a controller connection (e.g. during backoff), so rc->vconn
may be null and thus vconn_get_version(rc->vconn) dereferences a null
pointer.

Fixing the problem in a local way would require the rconn to remember the
previous version negotiated, and that fails if the rconn hasn't yet
connected or if the next connection negotiates a new version.

This commit instead adds the ability to a vconn to accept any OpenFlow
message version and modifies "ovs-ofctl snoop" to use that feature, thus
removing the need to negotiate the "correct" version on snoops.

Bug #14265.
Reported-by: Pratap Reddy <preddy@nicira.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Ethan Jackson <ethan@nicira.com>