blob: skip size check on possibly zero-filled records

While investigating elliptics startup failure due to corrupted index
we've found following index entry:

0000000: 0000 0000 0000 0000 0000 0000 0000 0000  ................
0000010: 0000 0000 0000 0000 0000 0000 0000 0000  ................
0000020: 0000 0000 0000 0000 0000 0000 0000 0000  ................
0000030: 0000 0000 0000 0000 0000 0000 0000 0000  ................
0000040: 0000 0000 0000 0000 0000 0000 0000 0000  ................
0000050: 0000 0000 0000 0000 0000 0000 0000 0000  ................

It happens that some versions of eblob (possibly even current one) are
leaving zero-filled index entries. Those index entries are likely to be
produced by `kill -9`.

Let's just ignore them for now (logging advise to run eblob_merge on given
base) thus allowing iterator and index initialization to proceed.

Reported by: izhidkov@
blob: we shouldn't ignore index corruption on iteration

This will fix self-healing of eblob in presence of broken unsorted
index.