Problem:
The calculation of `len` in `make_filter_cmd` for powershell falls short
by one character for the following ex command:
:%w !sort
This command satisfies these conditions:
- `itmp` is not null
- `otmp` is null
__NOTE__ that other shells circumvent this bug only because of `len`
allocation for six extra characters: a pair of curly braces and four
spaces:
cfdb4cbada/src/nvim/ex_cmds.c (L1551-L1554)
If allocation for these six characters are removed, then bash also faces
the same bug.
Solution:
Add allocation for 6 extra bytes. 1 would do, but let's keep powershell
in sync with other shells as much as possible.
2f1fd15554