Fix null pointer dereference.

In the function dump_thread_info, if /proc/<PID>/comm is unreadable, then
threadname will be used in a strncmp causing a crash. The fix is to
avoid the check if threadname is null.

Bug: 28615417
(cherry picked from commit 039976e5)

Change-Id: I1e61431b1549ecfdc49a72cbf193a364069cda79