FindDecoderBugs.java fails intermittently with GB18030 charset

The following test failed in the JDK12 CI:

sun/nio/cs/FindDecoderBugs.java

Here is a snippet from the log file:

-----
Results differ for direct buffers: GB18030
    "??"[4/4] => UNDERFLOW "u????"[1/19]
    "??"[0/4] => MALFORMED[4] ""[0/19] (direct)
failures=1
-----
Results differ for direct buffers: GB18030
    "??"[4/4] => UNDERFLOW "u????"[1/1]
    "??"[0/4] => MALFORMED[4] ""[0/1] (direct)
failures=1

Passed = 521138590, failed = 2

----------System.err:(12/717)----------
java.lang.AssertionError: Some tests failed
at FindDecoderBugs.main(FindDecoderBugs.java:448)
at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.base/java.lang.reflect.Method.invoke(Method.java:566)
at com.sun.javatest.regtest.agent.MainActionHelper$SameVMRunnable.run(MainActionHelper.java:229)
at java.base/java.lang.Thread.run(Thread.java:834)

JavaTest Message: Test threw exception: java.lang.AssertionError
JavaTest Message: shutting down test

result: Failed. Execution failed: `main' threw exception: java.lang.AssertionError: Some tests failed


test result: Failed. Execution failed: `main' threw exception: java.lang.AssertionError: Some tests failed