lib: fix bug in if_cmp_name_func()

lib: fix bug in if_cmp_name_func()

If the p1 and p2 arguments pointed to identical strings ending with
a non-numeric character (e.g. "lo"), this function would return -1
instead of 0 as one would expect. This inconsistency didn't matter
for sorted linked-lists but for red-black trees it's a major source
of problems.

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>