M_PREPEND() with an argument of M_TRYWAIT can fail, meaning the returned mbuf can be NULL.  Check for NULL in rip_output() when prepending an IP header.  This prevents mbuf exhaustion from causing a local kernel panic when sending raw IP packets.
M_PREPEND() with an argument of M_TRYWAIT can fail, meaning the
returned mbuf can be NULL.  Check for NULL in rip_output() when
prepending an IP header.  This prevents mbuf exhaustion from
causing a local kernel panic when sending raw IP packets.

PR:		kern/55886
Reported by:	Pawel Malachowski <pawmal-posting@freebsd.lublin.pl>
MFC after:	3 days

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