Show error instead of SEGV when System39.ini is missing

See #18.

src/cmd2F.c

@@ -1326,8 +1326,19 @@ void commands2F7B() {
 }
 
 void commands2F7C() {
+	if (!nact->files.ain) {
+		if (nact->files.gr_fname) {
+			SYSERROR("System39.ain is needed to run this game. Make sure you have an \"Ain\" line in %s.\n", nact->files.gr_fname);
+		} else {
+			SYSERROR("Ain file is needed to run this game. Make sure you have System39.ain file in the game directory.\n");
+		}
+	}
+
 	int index = sys_getdw();
-	
+	if (index >= nact->ain.msgnum) {
+		WARNING("message id out of bounds (%d)\n", index);
+		return;
+	}
 	sys_addMsg(nact->ain.msg[index]);
 	msgskip_onAinMessage(index);
 
@@ -1359,7 +1370,7 @@ void commands2F7F() {
 	sys_addMsg(svar_get(p1));
 	msgskip_onAinMessage(index);
 
-	DEBUG_COMMAND("2F7F %d, %d(%s,%s):\n", index, p1, nact->ain.msg[index], svar_get(p1));
+	DEBUG_COMMAND("2F7F %d, %d(%s):\n", index, p1, svar_get(p1));
 }
 
 void commands2F80() {

src/gameresource.c

@@ -193,6 +193,7 @@ static boolean initGameResourceFromFile(GameResource *gr, FILE *fp, const char *
 			goto errexit;
 		}
 	}
+	gr->gr_fname = strdup(gr_fname);
 	return TRUE;
 
  errexit:

src/gameresource.h

@@ -29,6 +29,7 @@
 #include "savedata.h"
 
 typedef struct {
+	const char *gr_fname;
 	const char *game_fname[DRIFILETYPEMAX][DRIFILEMAX];
 	int cnt[DRIFILETYPEMAX];
 	const char *save_path;