Fix compression failure on incompressible data

Fix compression failure on incompressible data

If the destination buffer is the minimum allowed size in
`ZSTD_compressSequences()` (2^17), then if the block isn't compressible
compression might fail with `dstSize_tooSmall`, when it should instead emit
a raw uncompressed block.

Additionally, `ZSTD_compressLiterals()` implicitly called
`ZSTD_noCompressLiterals()` if Huffman compression failed. Make that
explicit.