systemd: network: don't return allocated buffer of zero length from deserialize_in_addrs()

Imported from systemd:

    deserialize_in_addrs() allocates the buffer before trying to parse
    the IP address. Since a parsing error is silently ignored, the returned
    size might be zero. In such a case we shouldn't return any buffer.

    Anyway, there was no leak, because there are only two callers like

        r = deserialize_in_addrs(&lease->dns, dns);

    which both keep the unused buffer and later release it.

    Note that deserialize_in_addrs() doesn't free the pointer before
    reassigning the new output. The caller must take care to to pass
    "ret" with an allocated buffer that would be leaked when returning
    the result.

c24b682162

src/systemd/src/libsystemd-network/network-internal.c

@@ -456,7 +456,7 @@ int deserialize_in_addrs(struct in_addr **ret, const char *string) {
                 size++;
         }
 
-        *ret = TAKE_PTR(addresses);
+        *ret = size > 0 ? TAKE_PTR(addresses) : NULL;
 
         return size;
 }