fix undefined behavior with certain left-shift count values

Change-Id: I1b71de28312a56a3b1d27fcfaf0a7b71d5777e6c

libnetutils/ifc_utils.c

@@ -224,6 +224,11 @@ in_addr_t get_ipv4_netmask(int prefix_length)
 {
     in_addr_t mask = 0;
 
+    // C99 (6.5.7): shifts of 32 bits have undefined results
+    if (prefix_length == 0) {
+        return 0;
+    }
+
     mask = ~mask << (32 - prefix_length);
     mask = htonl(mask);