Fixes valgrind uninitialized data error (#129)

diff --git a/src/aarch64/simulator-aarch64.h b/src/aarch64/simulator-aarch64.h
index 5d6d975..e52d7f1 100644
--- a/src/aarch64/simulator-aarch64.h
+++ b/src/aarch64/simulator-aarch64.h
@@ -5360,7 +5360,7 @@
                          std::function<void(Simulator*, const Instruction*)>>;
   static const FormToVisitorFnMap* GetFormToVisitorFnMap();
 
-  uint32_t form_hash_;
+  uint32_t form_hash_{};
 
   static const PACKey kPACKeyIA;
   static const PACKey kPACKeyIB;