Fixes a corruption error when using LZ4.

LZ4 is capable of returning a value that gets assigned to cbytes that is equal
to neblock. In this case, uppon decompression the data that LZ4 poorly
compressed would simply be memcpyed instead of being run though LZ4.
(see blosc.c:600)

Adding this check and simply memcpying the data if this is the case fixes the issue.