fix(powershell): wrong length allocation for ":%w !" #20530

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.

src/nvim/ex_cmds.c

@@ -1382,7 +1382,7 @@ char *make_filter_cmd(char *cmd, char *itmp, char *otmp)
                                   : 0;
 
   if (itmp != NULL) {
-    len += is_pwsh  ? strlen(itmp) + sizeof("& { Get-Content " " | & " " }") - 1
+    len += is_pwsh  ? strlen(itmp) + sizeof("& { Get-Content " " | & " " }") - 1 + 6  // +6: #20530
                     : strlen(itmp) + sizeof(" { " " < " " } ") - 1;
   }
   if (otmp != NULL) {