contrib: fix IPv6 support in PJSIP ioqueue sendto.

PJSIP pj_ioqueue_sendto() fails in exception if addr argument
is an IPv6 address and if there is a pending write.
This is due to an PJ_ASSERT_RETURN() check not ported for IPv6.

This patch adds this support to the check and change
the internal write_operation structure to support generic IP addresses.

Change-Id: I966ef74c807995149d1db367188610374650dac0
Reviewed-by: Stepan Salenikovich <stepan.salenikovich@savoirfairelinux.com>