patch 9.1.1331: Leaking memory with cmdcomplete()

Problem:  Leaking memory with cmdcomplete()
          (zeertzjq, after v9.1.1329)
Solution: free the memory (Girish Palya)

closes: #17190

Signed-off-by: Girish Palya <girishji@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

src/testdir/test_autocmd.vim

@@ -2038,6 +2038,12 @@ func Test_Cmdline_Trigger()
   call assert_equal('', g:log)
   call feedkeys(":echo hello", "tx")
   call assert_equal('CmdlineLeavePre', g:log)
+  let g:count = 0
+  autocmd CmdlineLeavePre * let g:count += 1
+  call feedkeys(":let c = input('? ')\<cr>B\<cr>", "tx")
+  call assert_equal(2, g:count)
+  unlet! g:count
+  unlet! g:log
   bw!
 endfunc