chan_sip: Fix lastrtprx always updated

Packets are read regulary, when there is no data in buffer fr->frametype
is AST_FRAME_NULL. There was no check of frametype and lastrtprx always 
updated and, therefore, rtptimeout did not work at all.

ASTERISK-25270 #close

Change-Id: If3b5ca0dbb822582a86eb7d01dcae4e83448c41d