Fix buffer overrun in ZSTD_loadEntropy()

Fix buffer overrun in ZSTD_loadEntropy()

The table log set by `FSE_readNCount()` was not checked in
`ZSTD_loadEntropy()`.  This caused `FSE_buildDTable(dctx->MLTable, ...)`
to overwrite the beginning of `dctx->hufTable`.

The benchmarks look good, there is no obvious performance regression:

  > ./zstds/zstd.opt.0 -i10 -b1 -e5 ~/bench/silesia.tar
   1#silesia.tar       : 211988480 ->  73656930 (2.878), 268.2 MB/s , 701.0 MB/s
   2#silesia.tar       : 211988480 ->  70162842 (3.021), 199.5 MB/s , 666.9 MB/s
   3#silesia.tar       : 211988480 ->  66997986 (3.164), 154.9 MB/s , 655.6 MB/s
   4#silesia.tar       : 211988480 ->  66002591 (3.212), 128.9 MB/s , 648.4 MB/s
   5#silesia.tar       : 211988480 ->  65008480 (3.261),  98.4 MB/s , 633.4 MB/s

  > ./zstds/zstd.opt.2 -i10 -b1 -e5 ~/bench/silesia.tar
   1#silesia.tar       : 211988480 ->  73656930 (2.878), 266.1 MB/s , 703.7 MB/s
   2#silesia.tar       : 211988480 ->  70162842 (3.021), 199.0 MB/s , 666.6 MB/s
   3#silesia.tar       : 211988480 ->  66997986 (3.164), 156.2 MB/s , 656.2 MB/s
   4#silesia.tar       : 211988480 ->  66002591 (3.212), 133.2 MB/s , 647.4 MB/s
   5#silesia.tar       : 211988480 ->  65008480 (3.261),  96.3 MB/s , 633.3 MB/s