Zero out a local variable also when PURIFY is not defined.
This silence a warning brought up by valgrind whenever if_nametoindex
is used. This was already discussed in PR 166483, but the code
committed in r234329 guards the initilization with #ifdef PURIFY.
Therefore, valgrind still complains. Since this code is not performance
critical, always zero out the local variable to silence valgrind.

PR: 166483
Discussed with: eadler@
MFC after: 4 weeks