Don't crash if the stack state is empty.
Don't crash if the stack state is empty.

This only happens when throwables override fillInStackTrace().

Change-Id: I4513b53491e293725ba9e914228bdc19526ab874

treadstoneproject/artinst,treadstoneproject/artinst,treadstoneproject/artinst,treadstoneproject/artinst,treadstoneproject/artinst