Fix a bug when dumping locks from signal catcher

We dump the threads that are holding some important locks in the signal
catcher on a SIGQUIT. The check missed dumping them when a mutator lock
is exlcusively held but other locks aren't.

Test: art/test.py
Change-Id: If87c2d74ba3244d038fbc5e3a88730d36debb1e0