[isolate] Initialize module callbacks to nullptr

Bug: chromium:854476
Change-Id: I5f477327f303732c3482c47141b794c8d77d9786
Reviewed-on: https://chromium-review.googlesource.com/1108340Reviewed-by: Georg Neis <neis@chromium.org>
Commit-Queue: Sathya Gunasekaran <gsathya@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53896}

src/isolate.cc

@@ -2517,6 +2517,8 @@ Isolate::Isolate()
       atomics_wait_callback_(nullptr),
       atomics_wait_callback_data_(nullptr),
       promise_hook_(nullptr),
+      host_import_module_dynamically_callback_(nullptr),
+      host_initialize_import_meta_object_callback_(nullptr),
       load_start_time_ms_(0),
       serializer_enabled_(false),
       has_fatal_error_(false),