do not proceed with parsing after ccGetSome retuns empty ByteString

This patch is intended to fix #11. The motivation for the change
is that underlying implementation for ccGetSome is a recv/read call
which should return empty result only in case of exception. Last
assumption may not hold for arbitrary ccGetSome implementation.
Merge pull request #12 from veprbl/fix11

do not proceed with parsing after ccGetSome retuns empty ByteString