Fix two minor parsing bugs in -S processing:
- An unquoted space is always a separator, even when not "in_arg".
- When a new destination buffer must be allocated during variable substitution, only copy data from the active buffer to the new one when we *are* "in_arg".
These were noticed when testing variable-substitution of variables
which have null values, and are not inside quoted strings...
MFC plans: after a few days, and re@ approval...