Don't kill subcontext if it's null

We don't initialize subcontext in microdroid. subcontext->pid() will
crash in such case.

Test: atest MicrodroidTests
Change-Id: Ifca65acededbf7f113d8bf16afbb8e2c1aa91b41
diff --git a/init/subcontext.cpp b/init/subcontext.cpp
index bb3967e..bd2bec5 100644
--- a/init/subcontext.cpp
+++ b/init/subcontext.cpp
@@ -381,6 +381,9 @@
 }
 
 void SubcontextTerminate() {
+    if (!subcontext) {
+        return;
+    }
     subcontext_terminated_by_shutdown = true;
     kill(subcontext->pid(), SIGTERM);
 }