Fix single-step in native thread

If we attempt to single-step in a thread that is not running Java
code, there is no current method on the stack. So we need to check
for null before trying to decode debug info.

Bug: 21320157

(cherry picked from commit 52f5f93873ec244320e05b033243c5c7a3ae40e2)

Change-Id: I961108071dd58a23f6327370b00c35a4cabe0aa7
1 file changed