Avoid decrementing when no Symbol was created in ~SignatureStream

We can trivially avoid calling into Symbol::decrement_refcount from ~SignatureStream when no Symbols were created, which is the case if _previous_name is vmSymbols::java_lang_Object(). This achieves a small gain by reducing calls to decrement_refcount from places like AdapterHandleLibrary::get_adapter which sees heavy traffic during startup.

Making (part of) decrement_refcount inlineable would have a similar effect, but grows the static size of the jvm somewhat.