freeze: If priority is invalid here, it's time to abort

freeze: If priority is invalid here, it's time to abort

ctdb_start_freeze() is called from ctdb_control_freeze() which fixes the
priority if it's 0 and return error if it's invalid.  Other callers of
ctdb_start_freeze() are internal to CTDB.  So if priority is invalid in
ctdb_start_freeze(), definitely something is seriously wrong.

Signed-off-by: Amitay Isaacs <amitay@gmail.com>
(cherry picked from commit 87716e8f504d659515d3dbcf93badbf106873bc8)

Conflicts:
	server/ctdb_freeze.c