patch 8.1.0542: shiftwidth() does not take 'vartabstop' into account

Problem:    shiftwidth() does not take 'vartabstop' into account.
Solution:   Use the cursor position or a position explicitly passed.
            Also make >> and << work better with 'vartabstop'. (Christian
            Brabandt)
diff --git a/src/edit.c b/src/edit.c
index 239881e..6b5bc0f 100644
--- a/src/edit.c
+++ b/src/edit.c
@@ -262,7 +262,6 @@
 #ifdef FEAT_SMARTINDENT
 static void ins_try_si(int c);
 #endif
-static colnr_T get_nolist_virtcol(void);
 #if defined(FEAT_EVAL)
 static char_u *do_insert_char_pre(int c);
 #endif
@@ -10681,9 +10680,14 @@
  * Get the value that w_virtcol would have when 'list' is off.
  * Unless 'cpo' contains the 'L' flag.
  */
-    static colnr_T
+    colnr_T
 get_nolist_virtcol(void)
 {
+    // check validity of cursor in current buffer
+    if (curwin->w_buffer == NULL
+	|| curwin->w_buffer->b_ml.ml_mfp == NULL
+	|| curwin->w_cursor.lnum > curwin->w_buffer->b_ml.ml_line_count)
+	return 0;
     if (curwin->w_p_list && vim_strchr(p_cpo, CPO_LISTWM) == NULL)
 	return getvcol_nolist(&curwin->w_cursor);
     validate_virtcol();