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.
diff --git a/src/ex_getln.c b/src/ex_getln.c
index a888ba4..369d6c3 100644
--- a/src/ex_getln.c
+++ b/src/ex_getln.c
@@ -7042,7 +7042,13 @@
 	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);