Force fallback source name for stripped bytecode.

Reported by Lyrth. #1319

src/lj_bcread.c

@@ -443,7 +443,7 @@ static int bcread_header(LexState *ls)
 #endif
   }
   if ((flags & BCDUMP_F_STRIP)) {
-    ls->chunkname = lj_str_newz(ls->L, ls->chunkarg);
+    ls->chunkname = lj_str_newz(ls->L, *ls->chunkarg == BCDUMP_HEAD1 ? "=?" : ls->chunkarg);
   } else {
     MSize len = bcread_uleb128(ls);
     bcread_need(ls, len);