patch 8.2.1985: crash when closing terminal popup with <Cmd> mapping

Problem:    Crash when closing terminal popup with <Cmd> mapping.
Solution:   Check b_term is not NULL. (closes #7294)
diff --git a/src/testdir/test_terminal.vim b/src/testdir/test_terminal.vim
index 137a8a6..8d68ceb 100644
--- a/src/testdir/test_terminal.vim
+++ b/src/testdir/test_terminal.vim
@@ -1225,6 +1225,18 @@
   call delete('Xopenterm')
 endfunc
 
+func Test_terminal_popup_with_cmd()
+  " this was crashing
+  let buf = term_start(&shell, #{hidden: v:true})
+  let s:winid = popup_create(buf, {})
+  tnoremap <F3> <Cmd>call popup_close(s:winid)<CR>
+  call feedkeys("\<F3>", 'xt')
+
+  tunmap  <F3>
+  exe 'bwipe! ' .. buf
+  unlet s:winid
+endfunc
+
 func Check_dump01(off)
   call assert_equal('one two three four five', trim(getline(a:off + 1)))
   call assert_equal('~           Select Word', trim(getline(a:off + 7)))