res_pjsip_transport_websocket: Fix crash on receiving large SIP packets

Incoming SIP packets larger than PJSIP_MAX_PKT_LEN were themselves
truncated before passing to pjsip_tpmgr_receive_packet, but the length
was passed unaltered, thus causing memory corruption and segfault.

ASTERISK-25122 #close

Change-Id: I608a6b6b7f229eacc33a0a7d771d18e27e5b08ab