[fpc-pascal] destructor TDecompressionStream.Destroy question

[David Mears]

from fpc/packages/fcl-base/src/inc/zstream.pp

I'm not really sure why this is in the destructor for tdecompressionstream.

  if FZRec.avail_in <> 0 then
    Source.Seek(-FZRec.avail_in, soFromCurrent);

my tdecompressionstream  is fed by a tideadecryptstream - when
tdecompressionstream is freed it causes that seek in the decrypt for
some reason? , which isn't supported by the tdecryptionstream and it
throws a EIDEAError - or is there a problem with the source stream? 

generally I feel that exceptions are somehow uncivilized, so avoiding
one so totally unnecessary  (in context, at least) would be appreciated.
 
Dave