patch 9.0.0064: confusing error when using "q:" in command line window

Problem:    Confusing error when using "q:" in command line window.
Solution:   Check for the situation and give a better error message.
            (closes #10756)

Author: brammool

src/errors.h

@@ -3306,3 +3306,7 @@ EXTERN char e_substitute_nesting_too_deep[]
 EXTERN char e_invalid_argument_nr[]
 	INIT(= N_("E1291: Invalid argument: %ld"));
 #endif
+#ifdef FEAT_CMDWIN
+EXTERN char e_cmdline_window_already_open[]
+	INIT(= N_("E1292: Command-line window is already open"));
+#endif

src/normal.c

@@ -7147,6 +7147,11 @@ nv_record(cmdarg_T *cap)
 #ifdef FEAT_CMDWIN
 	if (cap->nchar == ':' || cap->nchar == '/' || cap->nchar == '?')
 	{
+	    if (cmdwin_type != 0)
+	    {
+		emsg(_(e_cmdline_window_already_open));
+		return;
+	    }
 	    stuffcharReadbuff(cap->nchar);
 	    stuffcharReadbuff(K_CMDWIN);
 	}

src/testdir/test_cmdwin.vim

@@ -356,5 +356,14 @@ func Test_cmdwin_ctrl_bsl()
   call assert_equal('', getcmdwintype())
 endfunc
 
+func Test_cant_open_cmdwin_in_cmdwin()
+  try
+    call feedkeys("q:q::q\<CR>", "x!")
+  catch
+    let caught = v:exception
+  endtry
+  call assert_match('E1292:', caught)
+endfunc
+
 
 " vim: shiftwidth=2 sts=2 expandtab

src/version.c

@@ -735,6 +735,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    64,
 /**/
     63,
 /**/