patch 8.2.0241: crash when setting 'buftype' to "quickfix"

Problem:    Crash when setting 'buftype' to "quickfix".
Solution:   Check that error list is not NULL. (closes #5613)

src/quickfix.c

@@ -4520,7 +4520,7 @@ qf_fill_buffer(qf_list_T *qfl, buf_T *buf, qfline_T *old_last)
 	*dirname = NUL;
 
 	// Add one line for each error
-	if (old_last == NULL)
+	if (old_last == NULL || old_last->qf_next == NULL)
 	{
 	    qfp = qfl->qf_start;
 	    lnum = 0;

src/testdir/test_quickfix.vim

@@ -1628,6 +1628,13 @@ func Test_setqflist_invalid_nr()
   eval []->setqflist(' ', {'nr' : $XXX_DOES_NOT_EXIST})
 endfunc
 
+func Test_setqflist_user_sets_buftype()
+  call setqflist([{'text': 'foo'}, {'text': 'bar'}])
+  set buftype=quickfix
+  call setqflist([], 'a')
+  enew
+endfunc
+
 func Test_quickfix_set_list_with_act()
   call XquickfixSetListWithAct('c')
   call XquickfixSetListWithAct('l')

src/version.c

@@ -742,6 +742,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    241,
 /**/
     240,
 /**/