add missing null terminator in windows file path helper function

lib/std/child_process.zig

@@ -441,6 +441,7 @@ pub const ChildProcess = struct {
 
         const any_ignore = (self.stdin_behavior == StdIo.Ignore or self.stdout_behavior == StdIo.Ignore or self.stderr_behavior == StdIo.Ignore);
 
+        // TODO use CreateFileW here since we are using a string literal for the path
         const nul_handle = if (any_ignore)
             windows.CreateFile(
                 "NUL",

lib/std/os/windows.zig

@@ -968,6 +968,7 @@ pub fn sliceToPrefixedSuffixedFileW(s: []const u8, comptime suffix: []const u16)
     const end_index = start_index + try std.unicode.utf8ToUtf16Le(result[start_index..], s);
     if (end_index + suffix.len > result.len) return error.NameTooLong;
     mem.copy(u16, result[end_index..], suffix);
+    result[end_index + suffix.len] = 0;
     return result;
 }