extmod/modlwip: Add back support for empty IP addresses.

Prior to commit 628abf8f25 which added IPv6
support, binding a socket with

    s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
    s.bind(("", PORT))

was possible.  But, the empty string is not regarded as a valid IP address
by lwip.  This commit adds a special case for the empty IP string,
restoring the previous CPython-compatible behaviour.

Signed-off-by: Felix Dörre <felix@dogcraft.de>

extmod/modlwip.c

@@ -367,8 +367,16 @@ mp_uint_t lwip_parse_inet_addr(mp_obj_t addr_in, ip_addr_t *out_ip) {
     mp_obj_get_array_fixed_n(addr_in, 2, &addr_items);
     size_t addr_len;
     const char *addr_str = mp_obj_str_get_data(addr_items[0], &addr_len);
-    if (!ipaddr_aton(addr_str, out_ip)) {
-        mp_raise_ValueError(MP_ERROR_TEXT("invalid arguments"));
+    if (*addr_str == 0) {
+        #if LWIP_IPV6
+        *out_ip = *IP6_ADDR_ANY;
+        #else
+        *out_ip = *IP_ADDR_ANY;
+        #endif
+    } else {
+        if (!ipaddr_aton(addr_str, out_ip)) {
+            mp_raise_ValueError(MP_ERROR_TEXT("invalid arguments"));
+        }
     }
     return mp_obj_get_int(addr_items[1]);
 }