Make sure that numbers never overflow uint32_t in inet_network to
properly validate octets encountered in IPv4 addresses.

Avoid malloca in NSS networks file code because /etc/networks lines
can be arbitrarily long. Instead of handcrafting the input for
inet_network by adding ".0" octets if they are missing, just left shift
the result. Also, do not accept invalid entries, but ignore the line
instead.

Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>
Signed-off-by: Tobias Stoeckmann <tobias@stoeckmann.org>