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

lib/libc/net/if_nametoindex.c

@@ -70,9 +70,7 @@ if_nametoindex(const char *ifname)
 
 	s = _socket(AF_INET, SOCK_DGRAM | SOCK_CLOEXEC, 0);
 	if (s != -1) {
-#ifdef PURIFY
 		memset(&ifr, 0, sizeof(ifr));
-#endif
 		strlcpy(ifr.ifr_name, ifname, sizeof(ifr.ifr_name));
 		if (_ioctl(s, SIOCGIFINDEX, &ifr) != -1) {
 			_close(s);