Small issue in 'exprstat'

The code should not compute an instruction address before checking that
it exists. (Virtually no machine complains of computing an invalid
address, as long as the address is not used, but for ISO C that is
undefined behavior.)

lparser.c

@@ -1822,8 +1822,9 @@ static void exprstat (LexState *ls) {
     restassign(ls, &v, 1);
   }
   else {  /* stat -> func */
-    Instruction *inst = &getinstruction(fs, &v.v);
+    Instruction *inst;
     check_condition(ls, v.v.k == VCALL, "syntax error");
+    inst = &getinstruction(fs, &v.v);
     SETARG_C(*inst, 1);  /* call statement uses no results */
   }
 }