JavaBBinder::onTransact - log if env null

This can happen if ProcessState::self()->joinThreadPool() is called
from a thread that is started manually, for instance.

Fatal error is kept (would be SIGSEGV). We don't attach to the JVM
automatically here because it's a resource-intensive operation to
do automatically.

Bug: 225134223
Test: N/A
Change-Id: Ie75b3ee7591c7c07f79c567866f0d26570cf1a4c

core/jni/android_util_Binder.cpp

@@ -396,6 +396,9 @@ protected:
     {
         JNIEnv* env = javavm_to_jnienv(mVM);
 
+        LOG_ALWAYS_FATAL_IF(env == nullptr,
+                            "Binder thread started or Java binder used, but env null. Attach JVM?");
+
         ALOGV("onTransact() on %p calling object %p in env %p vm %p\n", this, mObject, env, mVM);
 
         IPCThreadState* thread_state = IPCThreadState::self();