patch 9.0.1011: ml_get error when using screenpos()

Problem:    ml_get error when using screenpos().
Solution:   Give an error for the line number. (closes #11661)
diff --git a/src/move.c b/src/move.c
index 7a9ae3e..954a137 100644
--- a/src/move.c
+++ b/src/move.c
@@ -1500,6 +1500,11 @@
 	return;
 
     pos.lnum = tv_get_number(&argvars[1]);
+    if (pos.lnum > wp->w_buffer->b_ml.ml_line_count)
+    {
+	semsg(_(e_invalid_line_number_nr), pos.lnum);
+	return;
+    }
     pos.col = tv_get_number(&argvars[2]) - 1;
     pos.coladd = 0;
     textpos2screenpos(wp, &pos, &row, &scol, &ccol, &ecol);