Make tests/checksum_test.c `-Wconversion -Wno-sign-conversion` compliant.
Make tests/checksum_test.c `-Wconversion -Wno-sign-conversion` compliant.

sourcefrog/librsync,librsync/librsync,librsync/librsync,dbaarda/librsync,librsync/librsync,sourcefrog/librsync,librsync/librsync,sourcefrog/librsync,dbaarda/librsync,dbaarda/librsync,sourcefrog/librsync,dbaarda/librsync