Stream state was not copied back from substream in pb_enc_submessage,
which caused garbage output if the stream callback modified the state.
Expanded tests to cover this problem.
Thanks to Paweł Pery for debugging and reporting this problem.
git-svn-id: https://svn.kapsi.fi/jpa/nanopb@1089 e3a754e5-d11d-0410-8d38-ebb782a927b9
