FastHprofHeap throws on unknown instanceID in idToInstanceNumber

[JacekLach]

I have a heap dump where I expect some classes were unloaded during runtime. Whatever the cause, there are class entries in the CLASS_DUMP segment that don't have a valid entry:

The 'non-fast' heap handles this by returning 0, but FastHprofHeap throws an exception and aborts analysis.
I believe both should return 0 here, as if the class instanceid doesn't have a valid entry it will not show up in the actual dump.

[aragozin]

Feel free to suggest a patch. I agree that FastHprofHeap and HprofHeap should have similar behavior.