Only free res iff it's been allocated before. Re #1201

This patch fixes the segfault, so closing the ticket might be possible.
It's a very conservative patch. Perhaps the freeaddrinfo spec says that
it will never allocate res in case of errors, but for now, I neither
have the spec nor the will to rely on it.

This commit was SVN r17150.

opal/util/if.c

@@ -938,10 +938,13 @@ int opal_ifaddrtoname(const char* if_addr, char* if_name, int length)
     memset(&hints, 0, sizeof(hints));
     hints.ai_family = PF_UNSPEC;
     hints.ai_socktype = SOCK_STREAM;
+    res = NULL;
     error = getaddrinfo(if_addr, NULL, &hints, &res);
 
     if (error) {
-        freeaddrinfo (res);
+        if (NULL != res) {
+            freeaddrinfo (res);
+        }
         return OPAL_ERR_NOT_FOUND;
     }