Fix UB left shift of negative number

Author: bnoordhuis

quickjs.c

@@ -34907,7 +34907,7 @@ static int JS_WriteBigNum(BCWriterState *s, JSValueConst obj)
  e = a->expn + 3;
  else
  e = a->expn;
- e = (e << 1) | a->sign;
+ e = (e * 2) | a->sign;
  if (e < INT32_MIN || e > INT32_MAX) {
  JS_ThrowInternalError(s->ctx, "bignum exponent is too large");
  return -1;