BaseTools: Fix the build report crash issue
    
    In the following corner case, the build report
    will crash. This patch is to fix this problem.
    
    Case:
    Multiple SKU are used and 2 more DynamicHii structure Pcds
    are set in dsc file under different SKU. And 1 more of those
    Pcds are not used in any INF file.
    
    Signed-off-by: Bob Feng <bob.c.feng@intel.com>
    Cc: Liming Gao <gaoliming@byosoft.com.cn>
    Cc: Yuwei Chen <yuwei.chen@intel.com>
    Reviewed-by: Liming Gao <gaoliming@byosoft.com.cn>
    Reviewed-by: Yuwei Chen<yuwei.chen@intel.com>