Shenandoah: SEGV on accessing cset bitmap for NULL ptr

This seems to happen more or less reliably with:

CONF=linux-x86_64-server-fastdebug make images run-test TEST=gc/stress/gcbasher/TestGCBasherWithShenandoah.java TEST_VM_OPTS="-XX:-UseCompressedOops"

The full hs_err for such a failure is attached. The brief analysis of the failure:

siginfo: si_signo: 11 (SIGSEGV), si_code: 2 (SEGV_ACCERR), si_addr: 0x00007f783c1ad3a0


 ;; B9: # B970 B10 <- B8 Freq: 0.500054

  0x00007f783bc1950d: mov 0x18(%rax,%r12,8),%r12
  0x00007f783bc19512: testb $0x1,0x20(%r15)
  0x00007f783bc19517: jne 0x00007f783bc1d422 <--- enter LRB midpath

...

 ;; B970: # B10 B971 <- B9 Freq: 0.000500047

  0x00007f783bc1d422: mov %r12,%r10 <--- LRB midpath starts
  0x00007f783bc1d425: shr $0x13,%r10
  0x00007f783bc1d429: movabs $0x7f783c1ad3a0,%r11 <--- biased in-cset bitmap
  0x00007f783bc1d433: cmpb $0x0,(%r11,%r10,1) <--- in-cset check, SEGV here
  0x00007f783bc1d438: je 0x00007f783bc1951d

R10=0x0 is NULL
R11=0x00007f783c1ad3a0 is an unknown value

So, this is a SEGV in LRB midpath caused by trying to check the in-cset bit for NULL pointer. This means LRB path misses the null-check.

I think we missed this during initial LRB work, see the disassembly here:
  https://mail.openjdk.java.net/pipermail/shenandoah-dev/2019-March/008995.html

There are implicit null checks when accessing the object fields off the affected reference, but in-cset check is not covered by it. We are mostly lucky it always points to readable memory: in that case, whatever in-cset check replies, it would fail the next implicit NPE. That is, until we are unlucky, and in-cset(NULL) points to unmapped memory and SEGVs.