Fix a race in rename when relocking the source namecache entry. Since we

Fix a race in rename when relocking the source namecache entry.  Since we
may have blocked previously it is possible for the namecache entry to become
invalid (not destroyed since we hold a ref, but invalid).  For example, if
the source was removed.  This case only occurs when rename() is racing
against a remove() or another rename that is overwriting the target that
represents our 'from' name.

The race resulted in a NULL pointer dereference.

Reported-by: =?ISO-8859-2?Q?Toma=BE_Bor=B9tnar?= <tomaz.borstnar@amis.net>