fix broken check in fsx.c that clang complains about
fix broken check in fsx.c that clang complains about

fsx.c:999:25: error: comparison of unsigned expression < 0 is always false
      [-Werror,-Wtautological-compare]
                        if (progressinterval < 0)
                            ~~~~~~~~~~~~~~~~ ^ ~

eighthave/libsqlfs,guardianproject/libsqlfs,guardianproject/libsqlfs,eighthave/libsqlfs,guardianproject/libsqlfs,guardianproject/libsqlfs,eighthave/libsqlfs,guardianproject/libsqlfs,eighthave/libsqlfs,eighthave/libsqlfs