RFR (XS): 8244571: assert(!_thread->is_pending_jni_exception_check()) failed: Pending JNI Exception Check during class loading

[serguei.spit...@oracle.com]

Please, review fix for:
  https://bugs.openjdk.java.net/browse/JDK-8244571

Webrev:
  http://cr.openjdk.java.net/~sspitsyn/webrevs/2020/8244571-jvmti-test-jnicheck.1/

Summary:
  There are two places in the native part of test that cause assert and WARNING with the -Xcheck:jni.
  The assert is because there is no check for pending exception after the call to:
     jni->CallBooleanMethod(klass, is_hid_mid);
  Using a JNI ExceptionCheck() after the call fixes the issue.

  The following call to the JVM TI function:
     err = jvmti->GetClassLoaderClasses(loader, &count, &loader_classes);
  produces the warning (with a java level stack trace): WARNING: JNI local refs: 94, exceeds capacity: 32
  It is because the GetClassLoaderClasses returns an array of local references to the loader classes.
  Using a JNI EnsureLocalCapacity() before the JVM TI call also fixes the issue.

Testing:
  Running the test test/hotspot/jtreg/serviceability/jvmti/HiddenClass locally.
  Will run a mach5 job as well.

Thanks,
Serguei