patch 8.2.4151: reading beyond the end of a line

Problem:    Reading beyond the end of a line.
Solution:   For block insert only use the offset for correcting the length.
diff --git a/src/ops.c b/src/ops.c
index a996802..e0fa344 100644
--- a/src/ops.c
+++ b/src/ops.c
@@ -529,24 +529,9 @@
 	}
 
 	if (has_mbyte && spaces > 0)
-	{
-	    int off;
+	    // avoid copying part of a multi-byte character
+	    offset -= (*mb_head_off)(oldp, oldp + offset);
 
-	    // Avoid starting halfway a multi-byte character.
-	    if (b_insert)
-	    {
-		off = (*mb_head_off)(oldp, oldp + offset + spaces);
-		spaces -= off;
-		count -= off;
-	    }
-	    else
-	    {
-		// spaces fill the gap, the character that's at the edge moves
-		// right
-		off = (*mb_head_off)(oldp, oldp + offset);
-		offset -= off;
-	    }
-	}
 	if (spaces < 0)  // can happen when the cursor was moved
 	    spaces = 0;