Add CHECK to verify assumptions in the code.

On the main thread, the siginfo pointer will never be nullptr.
Add a CHECK to make sure this is true.

Test: Unit tests pass both 32 bit and 64 bit.
Test: Ran with debug.debuggerd.translate_proto_to_text set to 0
Test: to exercise old path.
Change-Id: I9d5ed0de5d652de8a4f9cd85eb57cbb1ec676404
2 files changed