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
diff --git a/debuggerd/libdebuggerd/tombstone.cpp b/debuggerd/libdebuggerd/tombstone.cpp
index ee64c3d..534d7be 100644
--- a/debuggerd/libdebuggerd/tombstone.cpp
+++ b/debuggerd/libdebuggerd/tombstone.cpp
@@ -36,6 +36,7 @@
 #include <string>
 
 #include <android-base/file.h>
+#include <android-base/logging.h>
 #include <android-base/properties.h>
 #include <android-base/stringprintf.h>
 #include <android-base/strings.h>
@@ -417,6 +418,8 @@
   }
 
   if (primary_thread) {
+    // The main thread must have a valid siginfo.
+    CHECK(thread_info.siginfo != nullptr);
     dump_probable_cause(log, unwinder, process_info, thread_info);
 
     dump_abort_message(log, unwinder->GetProcessMemory().get(), process_info.abort_msg_address);