[GR-20538] Refuse to start from multithreaded polyglot programs

PullRequest: graalpython/780

Author: msimacek

graalpython/com.oracle.graal.python/src/com/oracle/graal/python/PythonLanguage.java

@@ -584,6 +584,9 @@ protected boolean isThreadAccessAllowed(Thread thread, boolean singleThreaded) {
         if (singleThreaded) {
             return super.isThreadAccessAllowed(thread, singleThreaded);
         }
+        if (isWithThread == null) {
+            isWithThread = false;
+        }
         return isWithThread;
     }