Signature::basic_type should check length of signature Symbol

here's Signature code that unconditionally reads _body[0], which would assert (but class loading checks for invalid signatures that prevents this from happening)

BasicType Signature::basic_type(const Symbol* signature) {
  return basic_type(signature->char_at(0));
}

char Symbol::char_at(int index) const {
  assert(index >=0 && index < length(), "symbol index overflow");
  return (char)base()[index];
}

Signature::basic_type() should be fixed to either check for length, and/or assert that signature is a valid signature.