patch 8.2.1736: failure to compile a pattern not tested much

Problem:    Failure to compile a pattern not tested much.
Solution:   Add tests where a pattern fails to compile. (Yegappan Lakshmanan,
            closes #7004)

src/testdir/test_buffer.vim

@@ -138,6 +138,7 @@ func Test_bdelete_cmd()
   %bwipe!
   call assert_fails('bdelete 5', 'E516:')
   call assert_fails('1,1bdelete 1 2', 'E488:')
+  call assert_fails('bdelete \)', 'E55:')
 
   " Deleting a unlisted and unloaded buffer
   edit Xfile1