See added testcase 19.c from a bug report.  The problem is reading outside
the arguments buffers, even though we aren't allowed to.  The single
can_read_stream variable is not enough, sometimes we need to be able
to pop into outer contexts but not into arbitrarily outside contexts.

The trick is to terminate argument tokens with a EOF (and not just with
0 that makes us pop contexts), and deal with that in the few places
we have to,

This enables some cleanups of the can_read_stream variable use.