Fix of CVE: CVE-2022-0554, CVE-2022-0729, CVE-2022-0685, CVE-2022-0572

CVE-2022-0554: fix ending up with no current buffer - CVE-2022-0572: fix crashing when repeatedly using :retab - CVE-2022-0685: fix crashing when using special multi-byte character - CVE-2022-0729: fix crashing with specific regexp pattern and string...