[vm] Fix uninitialized Code::instructions_ during deferred loading.

TEST=ci
Bug: #59905
Bug: #60035
Change-Id: I733ee43f4e5e48ffa8eeb357de543233a1887b4a
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/407601
Commit-Queue: Ryan Macnak <[email protected]>
Reviewed-by: Brian Quinlan <[email protected]>

Author: rmacnak-google

runtime/vm/app_snapshot.cc

@@ -9412,6 +9412,8 @@ void Deserializer::ReadInstructions(CodePtr code, bool deferred) {
 #if defined(DART_PRECOMPILED_RUNTIME)
   if (deferred) {
     uword entry_point = StubCode::NotLoaded().EntryPoint();
+    // There are no serialized RawInstructions objects in this mode.
+    code->untag()->instructions_ = Instructions::null();
     code->untag()->entry_point_ = entry_point;
     code->untag()->unchecked_entry_point_ = entry_point;
     code->untag()->monomorphic_entry_point_ = entry_point;