ipv4: Fix ip4_addr_netmask_valid()

Use right API to convert from network to host byte order.  Address 0 is
not a valid netmask.

Author: sebhub

src/core/ipv4/ip4_addr.c

@@ -93,7 +93,11 @@ u8_t
 ip4_addr_netmask_valid(u32_t netmask)
 {
   u32_t mask;
-  u32_t nm_hostorder = lwip_htonl(netmask);
+  u32_t nm_hostorder = lwip_ntohl(netmask);
+
+  if (nm_hostorder == 0) {
+    return 0;
+  }
 
   /* first, check for the first zero */
   for (mask = 1UL << 31 ; mask != 0; mask >>= 1) {