patch 8.0.0706: crash when cancelling the cmdline window in Ex mode

Problem:    Crash when cancelling the cmdline window in Ex mode. (James McCoy)
Solution:   Do not set cmdbuff to NULL, make it empty.

Author: brammool

src/ex_getln.c

@@ -7042,7 +7042,13 @@ open_cmdwin(void)
 	else
 	    ccline.cmdbuff = vim_strsave(ml_get_curline());
 	if (ccline.cmdbuff == NULL)
+	{
+	    ccline.cmdbuff = vim_strsave((char_u *)"");
+	    ccline.cmdlen = 0;
+	    ccline.cmdbufflen = 1;
+	    ccline.cmdpos = 0;
 	    cmdwin_result = Ctrl_C;
+	}
 	else
 	{
 	    ccline.cmdlen = (int)STRLEN(ccline.cmdbuff);

src/version.c

@@ -764,6 +764,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    706,
 /**/
     705,
 /**/