crash_dump: don't abort if we fail to attach a sibling.

A TOCTOU race can occur between listing threads and attaching them.
Don't abort and leave the process in a stopped state when this happens.

Bug: http://b/34472671
Test: while true; do debuggerd -b `pidof audioserver`; done
Change-Id: Ib1632c3423fddf506b5c7874223c82fada78a85e