Allow label for SP800_108_Counter to include null bytes

[jrisc]

Null bytes to should allowed in the label argument of SP800_108_Counter() as this is required in some RFCs such as RFC8009 where this is explicitly stated in key derivation examples.

SP.800-108r1 does not give any indication about what the content of the label is expected to be. This is expected to be defined in the protocol specifications:

Label – A string that identifies the purpose for the derived keying material, which is encoded as a bit string. The encoding method for the Label is defined in a larger context, for example, in the protocol that uses a KDF.