Fix an infinite loop when parsing `V` headers

Fix an infinite loop when parsing `V` headers

Our tar header parsing tracks a count of bytes that need to be
consumed from the input.  After each header, we skip this many bytes,
discard them, and reset the count to zero.  The `V` header parsing
added the size of the `V` entry body to this count, but failed to
check whether that size was negative.  A negative size (from
overflowing the 64-bit signed number parsing) would decrement this
count, potentially leading us to consume zero bytes and leading to an
infinite loop parsing the same header over and over.

There are two fixes here:
* Check for a negative size for the `V` body
* Check for errors when skipping the bytes that
  need to be consumed

Thanks to Zhang Tianyi from Wuhan University for finding
and reporting this issue.