message: Added source uref read check
diff: @@ -96,6 +96,9 @@ pub trait Mint: RuntimeProvider + StorageProvider + SystemProvider {
  amount: U512,
  id: Option<u64>,
  ) -> Result<(), Error> {
  + if !source.is_readable() {
  + return Err(Error::InvalidAccessRights);
  + }
  if !source.is_writeable() || !target.is_addable() {
  return Err(Error::InvalidAccessRights);
  }
  