patch 9.0.0312: test for cmdheight zero fails

Problem:    Test for cmdheight zero fails.
Solution:   Do not close the messages window for CTRL-C.
diff --git a/src/normal.c b/src/normal.c
index 12b611e..926f504 100644
--- a/src/normal.c
+++ b/src/normal.c
@@ -6822,7 +6822,7 @@
     else if (no_reason)
     {
 #ifdef HAS_MESSAGE_WINDOW
-	if (popup_message_win_visible())
+	if (!cap->arg && popup_message_win_visible())
 	    popup_hide_message_win();
 	else
 #endif