res_pjsip: Use correct pool for storing the contact_user value.

When replacing the user portion of the Contact URI the code
was using the ephemeral pool instead of the tdata pool. This
could cause the Contact user value to become invalid after a
period of time.

The code will now use the tdata pool which persists for the
lifetime of the message instead.

ASTERISK-28794

Change-Id: I31e7b958e397cbdaeedd0ebb70bcf8dd2ed3c4d5