Fix: exception may have no traceback.

Author: fangerer

graalpython/com.oracle.graal.python/src/com/oracle/graal/python/builtins/modules/SysModuleBuiltins.java

@@ -295,7 +295,14 @@ public Object run(VirtualFrame frame,
                 PTraceback exceptionTraceback = getTracebackNode.execute(frame, exception);
                 // n.b. a call to 'sys.exc_info' always creates a new traceback with the current
                 // frame and links (via 'tb_next') to the traceback of the exception
-                PTraceback chainedTraceback = factory().createTraceback(escapedFrame, exceptionTraceback);
+                PTraceback chainedTraceback;
+                if (exceptionTraceback != null) {
+                    chainedTraceback = factory().createTraceback(escapedFrame, exceptionTraceback);
+                } else {
+                    // it's still possible that there is no traceback if, for example, the exception
+                    // has been thrown and caught and did never escape
+                    chainedTraceback = factory().createTraceback(escapedFrame, currentException);
+                }
                 exception.setTraceback(chainedTraceback);
                 return factory().createTuple(new Object[]{getClassNode.execute(exception), exception, chainedTraceback});
             }