tstools: force m_pts_length to be recalculated when m_pts_begin is set

m_pts_length can be correctly calculated (by calcEnd) only when
m_pts_begin is known, so calcBegin should have succeeded before
calcEnd is called.
Under certain circumstances, calcEnd could succeed before calcBegin,
leaving m_pts_length at an illegal value.
In that case, eDVBTSTools::calcLen might return an invalid length.

To avoid this, make sure the end value is invalidated, whenever
the begin value is set.
That ensures that begin and end are always calculated in the right
order, and m_pts_length will always be correct when m_begin_valid
and m_end_valid are both set.

thanks to betacentauri for finding this issue