comdlg32: Fix buffer overflow when current_filter is longer than MAX_PATH.
Wine-Bug: https://bugs.winehq.org/show_bug.cgi?id=57293
Without this change, the tests segfault due to a buffer overflow.
Note: This bug was surfaced but not caused by b5cbb556. Before b5cbb556, the savedlg test segfaults while the opendlg test does not. After it, both segfault.