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) {

test/functional/vimscript/system_spec.lua

@@ -662,4 +662,33 @@ describe('shell :!', function()
         1]])
     end
   end)
+
+  it(':{range}! without redirecting to buffer', function()
+    local screen = Screen.new(500, 10)
+    screen:attach()
+    insert([[
+      3
+      1
+      4
+      2]])
+    feed(':4verbose %w !sort<cr>')
+    if is_os('win') then
+      screen:expect{
+        any=[[Executing command: .?sort %< .*]]
+      }
+    else
+      screen:expect{
+        any=[[Executing command: .?%(sort%) %< .*]]
+
+      }
+    end
+    feed('<CR>')
+    helpers.set_shell_powershell(true)
+    feed(':4verbose %w !sort<cr>')
+    screen:expect{
+      any=[[Executing command: .?& { Get%-Content .* | & sort }]]
+    }
+    feed('<CR>')
+    helpers.expect_exit(command, 'qall!')
+  end)
 end)