bitshuffle: stop using uninitialized data as padding
bitshuffle: stop using uninitialized data as padding

The resize() incorrectly accounted for a block header that was never actually
written to data_. The result was that added padding was actually kHeaderSize
bytes "to the right", and the call to compress_lz4() read uninitialized data
from this part of data_ rather than the added padding.

What's the effect? Up to padding_bytes of uninitialized data gets
bitshuffled, compressed, and written to the block. At read time, it is
decompressed, debitshuffled, but ultimately ignored, as it was expected to
be just padded zeroes.

I observed this in cfile-test's TestMetadata built with MSAN
instrumentation, but oddly enough not in any other test in cfile-test, even
though others use bitshuffle and padding.

Change-Id: I25eba027ba356774173b2313c68436d7baddaddc
Reviewed-on: http://gerrit.cloudera.org:8080/15647
Reviewed-by: Andrew Wong <b68e4fdc6430321a6b47400732ff97d7ae91234e@cloudera.com>
Tested-by: Kudu Jenkins

helifu/kudu,helifu/kudu,helifu/kudu,helifu/kudu,helifu/kudu,helifu/kudu,helifu/kudu,helifu/kudu,helifu/kudu