vss: Propagate return value of afh_get_chunk().

vss: Propagate return value of afh_get_chunk().

With dynamic chunks, afh_get_chunk() may fail. Currently
vss_get_chunk() prints an error message in this case and returns
the null pointer. However, some callers of vss_get_chunk() happily
dereference the returned pointer without checking for NULL.

This patch modifies vss_get_chunk() to return int and teaches all
callers to check the return value. For the udp and dccp transport,
we disable the fec client temporarily in the error case while for
the http transport we log the error (but otherwise ignore it), and
try to continue with the next chunk.

This flaw was noticed by the clang static analyzer.