Fix error generation in load*.

Reported by Sergey Kaplun. #1353

src/lj_load.c

@@ -108,8 +108,9 @@ LUALIB_API int luaL_loadfilex(lua_State *L, const char *filename,
     copyTV(L, L->top-1, L->top);
   }
   if (err) {
+    const char *fname = filename ? filename : "stdin";
     L->top--;
-    lua_pushfstring(L, "cannot read %s: %s", chunkname+1, strerror(err));
+    lua_pushfstring(L, "cannot read %s: %s", fname, strerror(err));
     return LUA_ERRFILE;
   }
   return status;

src/lj_record.c

@@ -854,7 +854,10 @@ int lj_record_mm_lookup(jit_State *J, RecordIndex *ix, MMS mm)
       return 0;  /* No metamethod. */
     }
     /* The cdata metatable is treated as immutable. */
-    if (LJ_HASFFI && tref_iscdata(ix->tab)) goto immutable_mt;
+    if (LJ_HASFFI && tref_iscdata(ix->tab)) {
+      mix.tab = TREF_NIL;
+      goto immutable_mt;
+    }
     ix->mt = mix.tab = lj_ir_ktab(J, mt);
     goto nocheck;
   }