[AddressResolver] Add a check in shutdown to avoid some crashes if so… (project-chip#35824)

* [AddressResolver] Add a check in shutdown to avoid some crashes if someone calls it multiple times

* Apply suggestion from code review.

Co-authored-by: Andrei Litvin <andy314@gmail.com>

---------

Co-authored-by: Boris Zbarsky <bzbarsky@apple.com>
Co-authored-by: Andrei Litvin <andy314@gmail.com>

Author: 3 people

src/lib/address_resolve/AddressResolve_DefaultImpl.cpp

@@ -248,6 +248,10 @@ CHIP_ERROR Resolver::Init(System::Layer * systemLayer)
 
 void Resolver::Shutdown()
 {
+    // mSystemLayer is set in ::Init, so if it's null that means the resolver
+    // has not been initialized or has already been shut down.
+    VerifyOrReturn(mSystemLayer != nullptr);
+
     while (mActiveLookups.begin() != mActiveLookups.end())
     {
         auto current = mActiveLookups.begin();