slirp: fix structure initialization in tcp_listen()

A data structure of type sockaddr_in is allocated from stack but not
properly initialized. This may lead to a failure in the bind() call
later on. Fixed by filling the contents of the structure with zeroes
before using it.

Signed-off-by: Juha Riihimäki <juha.riihimaki@nokia.com>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
