When doping chunked multi-reads of a blob bigger than the max readsize,

When doping chunked multi-reads of a blob bigger than the max readsize,
a read can sometimes span beyond the end of file.

In which case individual smal read chunks beyond the end of file will return OK, 0 bytes of data and EOF==1

For this case, trap when len==0 and ignore these chunks.
Do not update the read-count if / when len is 0.