app_stack: Print proper exit location for PBXless channels.

app_stack: Print proper exit location for PBXless channels.

When gosub is executed on channels without a PBX, the context,
extension, and priority are initialized to the channel driver's
default location for that endpoint. As a result, the last Return
will restore this location and the Gosub logs will print out bogus
information about our exit point.

To fix this, on channels that don't have a PBX, the execution
location is left intact on the last return if there are no
further stack frames left. This allows the correct location
to be printed out to the user, rather than the bogus default
context.

ASTERISK-30076 #close

Change-Id: I1d42a99c9aa9e3708d32718863175158a894e414