BaseTools: Eliminate two shift-negative-value in FvLib.c
clang 3.8 flags -Wshift-negative-value warning, which turns fatal due to
use of -Werror.
Fixes: https://github.com/tianocore/edk2/issues/49
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Zenith432 <zenith432@users.sourceforge.net>
Reviewed-by: Liming Gao <liming.gao@intel.com>