files: trigger the Easter egg only when "zzy" is typed at the prompt

When the user mentions the file "zzy" on the command line,
the magic spell "Xyzzy" does not actually get typed, so the
Easter egg should then not be triggered.

src/files.c

@@ -2177,7 +2177,7 @@ int do_writeout(bool exiting, bool withprompt)
 		 * this is the first time we've done this, show an Easter
 		 * egg.  Display the credits. */
 		if (!did_credits && exiting && !ISSET(SAVE_ON_EXIT) &&
-								strcasecmp(answer, "zzy") == 0) {
+					openfile->filename[0] == '\0' && strcasecmp(answer, "zzy") == 0) {
 			if (LINES > 5 && COLS > 31) {
 				do_credits();
 				did_credits = TRUE;