libiptc: Fix for another segfault due to chain index NULL pointer

libiptc: Fix for another segfault due to chain index NULL pointer

Chain rename code missed to adjust the num_chains value which is used to
calculate the number of chain index buckets to allocate during an index
rebuild. So with the right number of chains present, the last chain in a
middle bucket being renamed (and ending up in another bucket) triggers
an index rebuild based on false data. The resulting NULL pointer index
bucket then causes a segfault upon reinsertion.

Closes: https://bugzilla.netfilter.org/show_bug.cgi?id=1713
Fixes: 64ff47cde38e4 ("libiptc: fix chain rename bug in libiptc")