Corrupt oop in ClassLoaderData

Very occasionally (but frequently enough to cause problems), we see a crash in the Serial collector when it's visiting oops in the `ClassLoaderData`. In these cases, the application is invoked with `-Xshare:on`. The stack trace looks like:

```
V [libjvm.so+0x6cba4c] DefNewGeneration::copy_to_survivor_space(oopDesc*)+0xac
V [libjvm.so+0x58869c] FastScanClosure::do_oop(oopDesc**)+0x8c
V [libjvm.so+0x60bb6a] ClassLoaderData::oops_do(OopClosure*, bool, bool)+0x7a
V [libjvm.so+0x6c9c8a] CLDScanClosure::do_cld(ClassLoaderData*)+0x3a
V [libjvm.so+0x60e3c9] ClassLoaderDataGraph::roots_cld_do(CLDClosure*, CLDClosure*)+0x39
V [libjvm.so+0x822beb] GenCollectedHeap::young_process_roots(StrongRootsScope*, OopsInGenClosure*, OopsInGenClosure*, CLDClosure*, OopStorage::ParState<false, false>*)+0x26b
V [libjvm.so+0x6cc8ed] DefNewGeneration::collect(bool, bool, unsigned long, bool)+0x4bd
V [libjvm.so+0x823c8d] GenCollectedHeap::collect_generation(Generation*, bool, unsigned long, bool, bool, bool, bool)+0x26d
V [libjvm.so+0x824548] GenCollectedHeap::do_collection(bool, bool, unsigned long, bool, GenCollectedHeap::GenerationType)+0x2e8
V [libjvm.so+0x8252b8] GenCollectedHeap::satisfy_failed_allocation(unsigned long, bool)+0x1e8
V [libjvm.so+0xf37f0f] VM_GenCollectForAllocation::doit()+0x8f
V [libjvm.so+0xf39547] VM_Operation::evaluate()+0xe7
V [libjvm.so+0xf3fa06] VMThread::evaluate_operation(VM_Operation*) [clone .constprop.67]+0xb6
V [libjvm.so+0xf3ff38] VMThread::loop()+0x428
V [libjvm.so+0xf403d3] VMThread::run()+0x73
```

It's crashing on `oop->size()`. The oop here is passed in RDI, RAX holds the `klass` pointer (which is null here):
```
RAX=0x0 is NULL
...
RDI=0x0000000800099120 is pointing into metadata
```
The metadata here is mapped to the shared archive:
```
800000000-800003000 rwxp 00001000 fe:00 543305 /var/lang/lib/server/runtime.jsa
800003000-80064b000 rw-p 00004000 fe:00 543305 /var/lang/lib/server/runtime.jsa
80064b000-80107f000 r--p 0064c000 fe:00 543305 /var/lang/lib/server/runtime.jsa
80107f000-801080000 rw-p 01080000 fe:00 543305 /var/lang/lib/server/runtime.jsa
801080000-801a90000 r--p 01081000 fe:00 543305 /var/lang/lib/server/runtime.jsa
```

Crash log file is attached. The environment here makes it difficult to collect core files or make changes to the command line parameters. I suspect it's an issue with CDS.