net: utils: Port parsing failure in net_ipaddr_parse()

If trying to parse a string like 192.0.2.2:80/foobar and
setting the length to 12 which means to parse the IP address
and port, the parsing failed because it used one extra character
from the string. This issue was not present if the input string
was terminated after the port number.

Add a test case to catch this problem.

Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>