Changes to remove compiler warnings on OSX build.

 1. cmScMeasCalcVal(): changed method of calculating the first and last
MIDI event in p->midiBuf[] and first and the score location window area
defined by 'bli' and 'ln'.

2. cmScMeasExec(): Set p->vsi and p->vsli to indicate that no new 'sets'
are available prior to the early return when locIdx==cmInvalidIdx.

be automatically applied to subsequent entries. New entries must therefore
only fill in the fields which change.