Frame Receivers can report more empty buffers than total buffers.

[ajgdls]

Reported in BC-684.

This has recently happened whilst testing a detector that was not in a fully functional state for the duration of the test.

Although not conclusive, it points to the fact that this problem is only noticed after some packets have been dropped (incomplete frames delivered). I'm not aware of this being reported by beamlines with fully operational detectors that are not dropping packets.