Fix undefined behavior in timestamp parsing

If this algorithm ever processed a string where the + came before the .,
then dot - plus would render a negative, and overflow, causing a crash.

In practice, this doesn't happen.

Tested: Inspection only at this time.  Difficult to trigger error.

Signed-off-by: Ed Tanous <edtanous@google.com>
Change-Id: Ic7f5153d144ac551118c4f4b2d61f82626ac3779