LanguageRuntime: ensure that we have a scratch context

We would previously not ensure that we had an initialised scratch
context before we would check if we encountered any fatal errors.
Protect against this case, while the experience may be degraded, this
prevents the debugger from terminating abnormally.

Author: compnerd

lldb/source/Plugins/LanguageRuntime/Swift/SwiftLanguageRuntimeDynamicTypeResolution.cpp

@@ -780,7 +780,7 @@ SwiftLanguageRuntimeImpl::GetMemberVariableOffsetRemoteAST(
     llvm::StringRef member_name) {
   auto scratch_ctx =
       instance_type.GetTypeSystem().dyn_cast_or_null<SwiftASTContext>();
-  if (scratch_ctx->HasFatalErrors())
+  if (scratch_ctx == nullptr || scratch_ctx->HasFatalErrors())
     return {};
 
   auto *remote_ast = &GetRemoteASTContext(*scratch_ctx);