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>