BaseTools: Correct the value assignment for StructurePcd

BaseTools: Correct the value assignment for StructurePcd

BZ: https://bugzilla.tianocore.org/show_bug.cgi?id=1752

This patch is to fix the code bug in StructurePcd overall
value assignment logic. If a Pcd Array size is fixed but the
size of actual value in Dsc or Dec is bigger than the Pcd
array size, the tool will report error about such setting and
stop build.

The patch is tested minplatform, Ovmf, structure pcd regression test
These tests are build pass. The patch also tested the following cases.
Our cases focused on PcdArraySize. 1.flexiable PcdArraySize.
 2.Fixed PcdArraySize, Pcd overall value exceeds the size of PcdArray.
 3.Fixed PcdArraySize, Pcd overall value Not exceeds the size of PcdArray.

Signed-off-by: Bob Feng <bob.c.feng@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>