netif_find: check if atoi means '0' or error

Since atoi() returns 0 on error, we need to check if name[2] is '0'.
If it's not, atoi() failed.

src/core/netif.c

@@ -1744,6 +1744,10 @@ netif_find(const char *name)
   }
 
   num = (u8_t)atoi(&name[2]);
+  if (!num && (name[2] != 0)) {
+    /* this means atoi has failed */
+    return NULL;
+  }
 
   NETIF_FOREACH(netif) {
     if (num == netif->num &&