K 10
svn:author
V 5
trasz
K 8
svn:date
V 27
2017-09-05T14:32:56.585077Z
K 7
svn:log
V 399
Make root_mount_rel(9) ignore NULL arguments, like it used to before r313351.
It would be better to fix API consumers to not pass NULL there - most of them,
such as gmirror, already contain the neccessary checks - but this is easier
and much less error-prone.

One known user-visible result is that it fixes panic on a failed "graid label".

PR:		221846
MFC after:	2 weeks
Sponsored by:	DARPA, AFRL

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