[Bug-tar] Potential write to null pointer

[Petr Sumbera]

Hi,

functions find_next_block() and start_header() seems to return at some 
cases NULL. But not all usages of this functions do check return values. 
Actually there are many cases where this is not the case.

E.g. See src/buffer.c and look for find_next_block() use.

While tar seems to work maybe there are some cases where we could expect 
nice crash. Any comments to this?

Thanks,

Petr