8156489: jjs tab-completion crashes with stack overflow error

Reviewed-by: jlaskey

nashorn/src/jdk.scripting.nashorn/share/classes/jdk/nashorn/tools/Shell.java

@@ -541,7 +541,7 @@ public class Shell implements PartialParser {
             @Override
             protected Expression assignmentExpression(final boolean noIn) {
                 exprStart[0] = this.start;
-                return super.expression();
+                return super.assignmentExpression(noIn);
             }
         };