patch 9.1.0132: "C" doesn't include composing chars with 'virtualedit'

Problem:  using "C" and 've=all' set, doesn't include composing chars
          when changing a line, keeps the composing chars for whatever
          is typed afterwards.
Solution: Use mb_head_off() and mb_ptr2len() instead of mb_tail_off().
          (zeertzjq)

closes: #14083

Signed-off-by: zeertzjq <zeertzjq@outlook.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/ops.c b/src/ops.c
index e66b4b5..57e6991 100644
--- a/src/ops.c
+++ b/src/ops.c
@@ -990,13 +990,20 @@
     static void
 mb_adjust_opend(oparg_T *oap)
 {
-    char_u	*p;
+    char_u	*line;
+    char_u	*ptr;
 
     if (!oap->inclusive)
 	return;
 
-    p = ml_get(oap->end.lnum);
-    oap->end.col += mb_tail_off(p, p + oap->end.col);
+    line = ml_get(oap->end.lnum);
+    ptr = line + oap->end.col;
+    if (*ptr != NUL)
+    {
+	ptr -= (*mb_head_off)(line, ptr);
+	ptr += (*mb_ptr2len)(ptr) - 1;
+	oap->end.col = ptr - line;
+    }
 }
 
 /*