[lldb] Fix nullptr dereference in AppleObjCRuntimeV2

Fix a potential nullptr dereference in AppleObjCRuntimeV2 by checking
the result of GetClassInfoUtilityFunction and returning a failure if
it's null.

The DynamicClassInfoExtractor was already doign the right thing, but the
SharedCacheClassInfoExtractor was missing this check.

Author: JDevlieghere

lldb/source/Plugins/LanguageRuntime/ObjC/AppleObjCRuntime/AppleObjCRuntimeV2.cpp

@@ -1995,6 +1995,11 @@ AppleObjCRuntimeV2::SharedCacheClassInfoExtractor::UpdateISAToDescriptorMap() {
   const uint32_t num_classes = 128 * 1024;
 
   UtilityFunction *get_class_info_code = GetClassInfoUtilityFunction(exe_ctx);
+  if (!get_class_info_code) {
+    // The callee will have already logged a useful error message.
+    return DescriptorMapUpdateResult::Fail();
+  }
+
   FunctionCaller *get_shared_cache_class_info_function =
       get_class_info_code->GetFunctionCaller();