Fix return values of frame-reading functions

Fix return values of frame-reading functions

This function was based on read_first_x that returned the number of
atoms, and was documented to do the same, but has always returned a
logical boolean about whether a frame has been read. This led to
aspects of gmx spatial and gmx trjcat -demux being broken.

Fixed by returning a proper bool, and fixing the remaining logic that
used the return value in a non-boolean sense.

Refs #2157

Change-Id: Ic871b56f68c7dbc654ab11b34ff82932353e6ceb