patch 8.0.1408: crash in setqflist()

Problem:    Crash in setqflist().
Solution:   Check for string to be NULL. (Dominique Pelle, closes #2464)

src/testdir/test_quickfix.vim

@@ -1428,6 +1428,11 @@ func XquickfixSetListWithAct(cchar)
   call assert_fails("call g:Xsetlist(list1, 0)", 'E928:')
 endfunc
 
+func Test_setqflist_invalid_nr()
+  " The following command used to crash Vim
+  call setqflist([], ' ', {'nr' : $XXX_DOES_NOT_EXIST})
+endfunc
+
 func Test_quickfix_set_list_with_act()
   call XquickfixSetListWithAct('c')
   call XquickfixSetListWithAct('l')
@@ -2946,6 +2951,15 @@ func Test_getqflist()
   call Xgetlist_empty_tests('l')
 endfunc
 
+func Test_getqflist_invalid_nr()
+  " The following commands used to crash Vim
+  cexpr ""
+  call getqflist({'nr' : $XXX_DOES_NOT_EXIST_XXX})
+
+  " Cleanup
+  call setqflist([], 'r')
+endfunc
+
 " Tests for the quickfix/location list changedtick
 func Xqftick_tests(cchar)
   call s:setup_commands(a:cchar)