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.