Stop testing negative timestamps -- they're not necessarily valid.

Stop testing negative timestamps -- they're not necessarily valid.

Both timevals and timespecs use a time_t for the seconds which POSIX
requires to be an "integer or real-floating type", so unsigned integer
types are perfectly valid.  (See FUSE for example -- though it's not
even using a time_t; at the moment it's just using uint64_t...).

Thanks to Tim Riemenschneider <t.riemenschneider@detco.de> for
reporting the problem.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>