dyndns_test: Use right socket length of for IPv4 address.
man inet_ntop says:
The caller specifies the number of bytes available in this buffer
in the argument size.
AF_INET
src points to a struct in_addr (in network byte order) which is
converted to an IPv4 network address in the dotted-decimal
format, "ddd.ddd.ddd.ddd". The buffer dst must be at least
INET_ADDRSTRLEN bytes long.
Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>
(cherry picked from commit e210ed5da220acebb6751db4466fe352de08eaeb)