patch 9.0.0875: using freed memory when executing delfunc at more prompt

Problem:    Using freed memory when executing delfunc at the more prompt.
Solution:   Check function list not changed in another place. (closes #11437)

src/testdir/test_functions.vim

@@ -3026,4 +3026,31 @@ func Test_virtcol()
   bwipe!
 endfunc
 
+func Test_delfunc_while_listing()
+  CheckRunVimInTerminal
+
+  let lines =<< trim END
+      set nocompatible
+      for i in range(1, 999)
+        exe 'func ' .. 'MyFunc' .. i .. '()'
+        endfunc
+      endfor
+      au CmdlineLeave : call timer_start(0, {-> execute('delfunc MyFunc622')})
+  END
+  call writefile(lines, 'Xfunctionclear', 'D')
+  let buf = RunVimInTerminal('-S Xfunctionclear', {'rows': 12})
+
+  " This was using freed memory.  The height of the terminal must be so that
+  " the next function to be listed with "j" is the one that is deleted in the
+  " timer callback, tricky!
+  call term_sendkeys(buf, ":func /MyFunc\<CR>")
+  call TermWait(buf, 50)
+  call term_sendkeys(buf, "j")
+  call TermWait(buf, 50)
+  call term_sendkeys(buf, "\<CR>")
+
+  call StopVimInTerminal(buf)
+endfunc
+
+
 " vim: shiftwidth=2 sts=2 expandtab