lz4hc.c: clamp compression levels > 12
lz4hc.c: clamp compression levels > 12

I noticed that, while 'lz4 -12' works fine, 'lz4 -13' does not compress
at all.

$ cat </etc/passwd |wc -c
2565
$ lz4 -12 </etc/passwd |wc -c
1456
$ lz4 -13 </etc/passwd |wc -c
2584

Perhaps the best way to fix this is to clamp the compression level
in the LZ4HC_compress_generic routine.

unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso