patch 8.2.5016: access before start of text with a put command

Problem:    Access before start of text with a put command.
Solution:   Check the length is more than zero.
diff --git a/src/register.c b/src/register.c
index 5ebaa05..d5a56ae 100644
--- a/src/register.c
+++ b/src/register.c
@@ -2219,9 +2219,12 @@
 	    len = STRLEN(y_array[y_size - 1]);
 	    col = (colnr_T)len - lendiff;
 	    if (col > 1)
-		curbuf->b_op_end.col = col - 1
-				- mb_head_off(y_array[y_size - 1],
+	    {
+		curbuf->b_op_end.col = col - 1;
+		if (len > 0)
+		    curbuf->b_op_end.col -= mb_head_off(y_array[y_size - 1],
 						y_array[y_size - 1] + len - 1);
+	    }
 	    else
 		curbuf->b_op_end.col = 0;