arm: Fix heap overflow issue in Neon64Load operation

This patch fixes an issue identified by ASAN where the Neon64Load
operation assumes the packet always contains 16 bytes.

Change-Id: If24a7e461d60cb80970dfbe61d923d7d56926698
Reviewed-by: Giacomo Gabrielli <giacomo.gabrielli@arm.com>
Reviewed-by: Curtis Dunham <curtis.dunham@arm.com>

src/arch/arm/isa/templates/neon64.isa

@@ -332,7 +332,8 @@ def template NeonLoadCompleteAcc64 {{
         %(op_decl)s;
         %(op_rd)s;
 
-        MemUnion &memUnion = *(MemUnion *)pkt->getPtr<uint8_t>();
+        MemUnion memUnion { { } };
+        memcpy(&memUnion, pkt->getPtr<uint8_t>(), pkt->getSize());
 
         if (fault == NoFault) {
             %(memacc_code)s;