Tidy logging code not using UNIMPLEMENTED.

Change-Id: I7a79c1671a6ff8b2040887133b3e0925ef9a3cfe
diff --git a/runtime/native/dalvik_system_VMRuntime.cc b/runtime/native/dalvik_system_VMRuntime.cc
index ec7d82d..c35bb30 100644
--- a/runtime/native/dalvik_system_VMRuntime.cc
+++ b/runtime/native/dalvik_system_VMRuntime.cc
@@ -326,6 +326,7 @@
       break;
     default:
       LOG(FATAL) << "Unreachable - invocation type: " << invoke_type;
+      UNREACHABLE();
   }
   if (method == NULL) {
     return;