Support Runtime::Abort() before the runtime has started.

This lets you get useful diagnostics out of LOG(FATAL) and CHECK(FATAL) and
so on even if the problem occurs really early on.

Change-Id: I2bfd906191d8817de719c3d38e7fa5de844bebba
diff --git a/src/runtime.cc b/src/runtime.cc
index 6cb874f..306bb87 100644
--- a/src/runtime.cc
+++ b/src/runtime.cc
@@ -109,6 +109,10 @@
     }
     gAborting = true;
     os << "Runtime aborting...\n";
+    if (Runtime::Current() == NULL) {
+      os << "(Runtime does not yet exist!)\n";
+      return;
+    }
     Thread* self = Thread::Current();
     if (self == NULL) {
       os << "(Aborting thread was not attached to runtime!)\n";