ripd: Fix crash when ip rip split-horizon poisoned-reverse is configed

ripd: Fix crash when ip rip split-horizon poisoned-reverse is configed

The code was attempting to access a variable that would always be NULL.
In fact this code has been broken since the rip ECMP changes
were put into place a few years back.

I'm going to come straight out and say that I don't fully
understand this code.  rinfo is the first item in the ecmp
list and tmp_rinfo is used to iterate over all the items
in the ecmp list.  It sure looks like that the changes
made here were just hacked together.  So I modified
the tmp_rinfo loop to just work on tmp_rinfo and
the check that was crashing I modified to just use
the rinfo since that what was checked originally
in code before the ECMP was added.  So consider
this a hack job to stop the crashing.

I think worse case is that we might be sending some routes
back out interfaces it shouldn't be if you have
ip rip split-horizon poisoned-reverse configured but
that is less bad(tm) than crashing.

Fixes: #1717
Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>