easter: show the crawl only when there is room enough for the lines

The crawl needs at least five rows to be perceived as a crawl, and
the widest line is 31 characters (ignoring translations, which might
be even longer).  Formerly, when only two or three rows were available,
nothing was shown at all, which was puzzling.  So, better do the credits
only when there is room enough for them, and print a message otherwise.

src/files.c

@@ -2169,8 +2169,11 @@ int do_writeout(bool exiting, bool withprompt)
 		 * egg.  Display the credits. */
 		if (!did_credits && exiting && !ISSET(TEMP_FILE) &&
 								strcasecmp(answer, "zzy") == 0) {
-			do_credits();
-			did_credits = TRUE;
+			if (LINES > 5 && COLS > 31) {
+				do_credits();
+				did_credits = TRUE;
+			} else
+				statusbar(_("Screen is too small"));
 			break;
 		}
 #endif