Update forbid-prop-types.js

handle situation where getFirstTokens() call returns only 1 Token

Author: Nuno Campos

lib/rules/forbid-prop-types.js

@@ -33,7 +33,7 @@ module.exports = function(context) {
     // (babel-eslint does not expose property name so we have to rely on tokens)
     if (node.type === 'ClassProperty') {
       var tokens = context.getFirstTokens(node, 2);
-      if (tokens[0].value === 'propTypes' || tokens[1].value === 'propTypes') {
+      if (tokens[0].value === 'propTypes' || (tokens[1] && tokens[1].value === 'propTypes')) {
         return true;
       }
       return false;