lib/LzmaEnc: Validate "len" before subtracting

lib/LzmaEnc: Validate "len" before subtracting

In LzmaEnc_CodeOneBlock(), both GetOptimumFast() and GetOptimum()
returns a value of greater or equal to 1, which is assigned to
"len". But since LZMA_MATCH_LEN_MIN == 2, "len" should be validated
before performing "len - LZMA_MATCH_LEN_MIN" to avoid underflow
when "len" equals to 1.

Fixes: CID 51508
Signed-off-by: Lidong Chen <lidong.chen@oracle.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>
Reviewed-by: Sudhakar Kuppusamy <sudhakar@linux.ibm.com>