CLSA-2026-1779130441 vim: Fix of 3 CVEs

CVE-2022-0261: fix heap-based buffer overflow in blockinsert in src/ops.c - CVE-2022-0318: fix heap-based buffer overflow in utfheadoff in mbyte.c - CVE-2022-3520: clamp bopend.col = 0 in doput to prevent Visual block put underflow...