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)

src/move.c

@@ -1500,6 +1500,11 @@ f_screenpos(typval_T *argvars UNUSED, typval_T *rettv)
 	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);

src/testdir/test_cursor_func.vim

@@ -167,6 +167,9 @@ func Test_screenpos_number()
   let pos = screenpos(winid, 1, 66)
   call assert_equal(winrow, pos.row)
   call assert_equal(wincol + 66 + 3, pos.col)
+
+  call assert_fails('echo screenpos(0, 2, 1)', 'E966:')
+
   close
   bwipe!
 endfunc

src/version.c

@@ -695,6 +695,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1011,
 /**/
     1010,
 /**/