Warn when thread state lookup fails.

Change-Id: I06781caddcade26148921bc225f28db8b3a63a35
diff --git a/runtime/native/java_lang_Thread.cc b/runtime/native/java_lang_Thread.cc
index 0b84005..86db893 100644
--- a/runtime/native/java_lang_Thread.cc
+++ b/runtime/native/java_lang_Thread.cc
@@ -88,6 +88,7 @@
     case kSuspended:                      return kJavaRunnable;
     // Don't add a 'default' here so the compiler can spot incompatible enum changes.
   }
+  LOG(ERROR) << "Unexpected thread state: " << internal_thread_state;
   return -1;  // Unreachable.
 }