message: fix crash when reading integer matrices with negative values
diff: @@ -212,7 +212,7 @@ where I: SpIndex,
  DataType::Integer => {
  let val = entry.next()
  .ok_or(BadMatrixMarketFile)
  - .and_then(|s| s.parse::<usize>()
  + .and_then(|s| s.parse::<isize>()
  .or(Err(BadMatrixMarketFile)))?;
  NumCast::from(val).unwrap()
  },
  