defensive coding: fix situations like zaach/jison#358 / handlebars-la…

…ng/handlebars.js#1368 in the spurious case where the lexer `parseError()` API is invoked *before* the `setInput()` API is invoked (which would *always* set up a non-NULL (though possibly *empty*) `this.yy` lexer context. (Original jison (https://github.com/zaach/jison) doesn't do the latter either, BTW)

regexp-lexer.js

@@ -1338,13 +1338,14 @@ function getRegExpLexerPrototype() {
             if (!ExceptionClass) {
                 ExceptionClass = this.JisonLexerError;
             }
-            if (this.yy.parser && typeof this.yy.parser.parseError === 'function') {
-                return this.yy.parser.parseError(str, hash, ExceptionClass) || this.ERROR;
-            } else if (typeof this.yy.parseError === 'function') {
-                return this.yy.parseError(str, hash, ExceptionClass) || this.ERROR;
-            } else {
-                throw new ExceptionClass(str, hash);
-            }
+            if (this.yy) {
+                if (this.yy.parser && typeof this.yy.parser.parseError === 'function') {
+                    return this.yy.parser.parseError(str, hash, ExceptionClass) || this.ERROR;
+                } else if (typeof this.yy.parseError === 'function') {
+                    return this.yy.parseError(str, hash, ExceptionClass) || this.ERROR;
+                } 
+            }
+            throw new ExceptionClass(str, hash);
         },
 
         /**