Replace the duplicated finishing code (set destination buffer length and
set return code to 0) in the case of decompressing a buffer with no header
with a goto to the success case of decompressing a buffer with a header.

This is a trivial change that allows both success cases to use common code,
and includes the pr_debug() msg in both cases as well.
Signed-off-by: Dan Streetman <ddstreet@ieee.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>