This would try reading from an uninitialized buffer thereby resulting
in a bad audio glitch. This does two things to fix the problem:

  1) When there are no input nodes attached to an input bus, nothing is
     read and 0 will be returned for the frames read variable.

  2) The buffer is silenced by default.

This fixes a bug with ma_engine where it would glitch in the moment
just after the engine is initialized and before a sound or group is
attached.