lib/common: warning: this statement may fall through

lib/common: warning: this statement may fall through

The following warning appears during the build. Fixed the review comments too.

zstd/lib/common/bitstream.h: In function ‘BIT_initDStream’:
zstd/lib/common/bitstream.h:277:33: warning: this statement may fall through [-Wimplicit-fallthrough=]
      case 7: bitD->bitContainer += (size_t)(((const BYTE*)(srcBuffer))[6]) <<
      (sizeof(bitD->bitContainer)*8 - 16);

Signed-off-by: Jos Collin <jcollin@redhat.com>