ART: JNI ExceptionDescribe crashes if no exception occurred

Some tests are calling ExceptionDescribe without checking if
we have an exception occurred. The most JVM's like Dalvik can
handle this in a good way, but art crashes with JNI error.

This adds a check in art::ExceptionDescribe for a case when it
called without exception.

Change-Id: Id9eddcc73e78b1197109be5a6340f9ff60940c74
Signed-off-by: Alexei Zavjalov <alexei.zavjalov@intel.com>
diff --git a/runtime/jni_internal.cc b/runtime/jni_internal.cc
index 083f179..0624281 100644
--- a/runtime/jni_internal.cc
+++ b/runtime/jni_internal.cc
@@ -684,6 +684,11 @@
   static void ExceptionDescribe(JNIEnv* env) {
     ScopedObjectAccess soa(env);
 
+    // If we have no exception to describe, pass through.
+    if (!soa.Self()->GetException(nullptr)) {
+      return;
+    }
+
     StackHandleScope<3> hs(soa.Self());
     // TODO: Use nullptr instead of null handles?
     auto old_throw_this_object(hs.NewHandle<mirror::Object>(nullptr));