chan_sip.c: Stop handling continuation lines after reading headers

lws2sws() does not stop trying to handle header continuation lines
even after all headers have been found. This is problematic if the
first character of a SIP message body is a space or tab character, so
we update to recognize the end of the message header.

ASTERISK-28693 #close
Reported by: Frank Matano

Change-Id: Idec8fa58545cd3fd898cbe0075d76c223f8d33df