fixed improper hint

fixed improper hint

when LZ4F_decompress() decodes an uncompressed block,
it provides an incorrect hint for next block
when frame checksum is enabled and block checksum is not.

Impact is low : the hint is just an hint,
the decoder works whatever the amount of input provided.

But the assumption that each call to LZ4F_decompress()
would generate just one complete block if input size hint was respected
was broken by this error.