res_pjsip_header_funcs: wrong pool used tdata headers

res_pjsip_header_funcs: wrong pool used tdata headers

When adding headers to an outgoing request the headers were cloned using
the dialog's pool when they should have been cloned using tdata's pool.
Under certain circumstances it was possible for the dialog object, and
its pool to be freed while tdata is still active and available. Thus the
cloned header "disappeared", and when tdata tried to later access it a
crash would occur.

This patch makes it so all added headers are cloned appropriately using
tdata's pool.

ASTERISK-29411 #close
ASTERISK-29535 #close

Change-Id: I9852025b5ee93ce1c038209150ee9dba1e0767c5