CLSA-2026-1779183103 vim: Fix of 6 CVEs

CVE-2021-3903: do not set VALIDBOTLINE in updatetopline when screen is invalid - CVE-2022-1616: tighten appendcommand loop bound + pre-write length check to avoid buffer overflow with composing chars - CVE-2022-2042: initialize attr in spellmoveto and capture emptyline before mlgetbuf invalidates...