Handle NULL return from class loader.

The VM was asserting that it couldn't happen, when it fact it can quite
easily if a class loader is broken.

Bug 2817346.

Change-Id: Id2a4edab428a0ef0fa9aabc8e4a7f6a4d614fb9e
diff --git a/vm/oo/Class.c b/vm/oo/Class.c
index 2776100..d708bb8 100644
--- a/vm/oo/Class.c
+++ b/vm/oo/Class.c
@@ -1292,8 +1292,11 @@
         dvmReleaseTrackedAlloc(excep, self);
         clazz = NULL;
         goto bail;
-    } else {
-        assert(clazz != NULL);
+    } else if (clazz == NULL) {
+        LOGW("ClassLoader returned NULL w/o exception pending\n");
+        dvmThrowException("Ljava/lang/NullPointerException;",
+            "ClassLoader returned null");
+        goto bail;
     }
 
     dvmAddInitiatingLoader(clazz, loader);