sip: release dialog after invite creation

PJSIP dialog is locked at creation during incoming call,
but invite structure lock it also and our lock wasn't released.

Add this dialog lock release right after invite creation.

This fix non-destroyed transport for incoming calls.

Change-Id: I5ad1bb0d6dd87e1a58f0a23cd93c85dac5fed5fa
Reviewed-by: Anthony Léonard <anthony.leonard@savoirfairelinux.com>