Fix CC DCHECK failure in 152-gc-and-run-finalization.

This fixes the second crash trace in 33389022#1.

Load the referent once which avoids passing nullptr to IsMarked().

It's still racey but it's okay because leaving a Reference with a
cleared referent gray is fine, if not optimal performance-wise.

Bug: 33389022
Bug: 12687968
Test: test-art-host with CC. 152 in a loop.

Change-Id: I2b389022175e38bdc40518b9553a2f5180dbc649