If the downloaded file is empty or of unknown size.

Correctly reject non-regular files: previously, the code would get
stuck on FIFOs, and ignore other types silently.

Check that reading the file actually succeeds.

This avoids reallocating and copying the block in block_Realloc() in
some cases.

Header size can reach UINT32_MAX.

Unlike stream_Size(), this helper distinguishes between unknown and
zero sizes.

Due to backward command line compatibility, zero size is not possible
with the old imem. Consider using the proper LibVLC API.

So far, it was returning zero on error. That prevented distinction
between an empty file and a file with unknown size.