Fix intermittent process monitor test failures (#1251)
Fix intermittent process monitor test failures (#1251)

Before this change, `_read_len` would not be reset to zero for the
stderr case, meaning that the next chunk of data would be read into a
wrong buffer offset (causing data corruption).

Signed-off-by: Malthe Borch <5eef1c07bd203d60699ccb45b46a933058dff478@gmail.com>
mkfifo/ponyc,jemc/ponyc,CausalityLtd/ponyc,mkfifo/ponyc,boemmels/ponyc,CausalityLtd/ponyc,sgebbie/ponyc,jupvfranco/ponyc,ponylang/ponyc,Perelandric/ponyc,sgebbie/ponyc,malthe/ponyc,mkfifo/ponyc,Perelandric/ponyc,dipinhora/ponyc,dipinhora/ponyc,kulibali/ponyc,malthe/ponyc,cquinn/ponyc,malthe/ponyc,cquinn/ponyc,Perelandric/ponyc,ponylang/ponyc,Praetonus/ponyc,dipinhora/ponyc,Theodus/ponyc,kulibali/ponyc,jupvfranco/ponyc,cquinn/ponyc,kulibali/ponyc,jupvfranco/ponyc,Theodus/ponyc,boemmels/ponyc,sgebbie/ponyc,jemc/ponyc,Praetonus/ponyc,sgebbie/ponyc,Perelandric/ponyc,Perelandric/ponyc,boemmels/ponyc,malthe/ponyc,Theodus/ponyc,jupvfranco/ponyc,Praetonus/ponyc,boemmels/ponyc,ponylang/ponyc,boemmels/ponyc,mkfifo/ponyc,jupvfranco/ponyc,Praetonus/ponyc,kulibali/ponyc,mkfifo/ponyc,cquinn/ponyc,sgebbie/ponyc,CausalityLtd/ponyc,jemc/ponyc,Theodus/ponyc,Theodus/ponyc