h52nx: reset 'n_frames_actual_bliss_scan' before iterating over all entries.

In some cases (autoflat) entries can have the expected detector dataset but this one is 1D (link to EDF files) instead of 3D. When happening the '._n_frames_actual_bliss_scan' was never set. As a consequence electric current for those 'unexisting frame' was created.

close #177