Update mpireports scripts to deal with size 0 messages

check_mpireports and match_mpireports need to strip the - sign from the size column in a recv message log. However, if the size of the message is 0, this - sign will not be present. Assuming the first character is a - and blindly stripping it leads to false positives in the error reporting. It is safer to use str.replace("-", "") which also works if the - sign is absent.