patch 9.1.1003: [security]: heap-buffer-overflow with visual mode

Problem:  [security]: heap-buffer-overflow with visual mode when
          using :all, causing Vim trying to access beyond end-of-line
          (gandalf)
Solution: Reset visual mode on :all, validate position in gchar_pos()
          and charwise_block_prep()

This fixes CVE-2025-22134

Github Advisory:
https://github.com/vim/vim/security/advisories/GHSA-5rgf-26wj-48v8

Co-authored-by: zeertzjq <zeertzjq@outlook.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/ops.c b/src/ops.c
index a75efab..9efef38 100644
--- a/src/ops.c
+++ b/src/ops.c
@@ -2586,6 +2586,7 @@
     colnr_T startcol = 0, endcol = MAXCOL;
     colnr_T cs, ce;
     char_u *p;
+    int	plen = ml_get_len(lnum);
 
     p = ml_get(lnum);
     bdp->startspaces = 0;
@@ -2646,7 +2647,7 @@
     else
 	bdp->textlen = endcol - startcol + inclusive;
     bdp->textcol = startcol;
-    bdp->textstart = p + startcol;
+    bdp->textstart = startcol <= plen ? p + startcol : p;
 }
 
 /*