[rst mode] Better innerMode implementation

Also fixes CodeMirror.innerMode to give up on null or fixpoint

Issue #1704

Author: marijnh

lib/codemirror.js

@@ -3358,6 +3358,7 @@ window.CodeMirror = (function() {
   CodeMirror.innerMode = function(mode, state) {
     while (mode.innerMode) {
       var info = mode.innerMode(state);
+      if (!info || info.mode == mode) break;
       state = info.state;
       mode = info.mode;
     }

mode/rst/rst.js

@@ -482,7 +482,9 @@ CodeMirror.defineMode('rst-base', function (config) {
         },
 
         innerMode: function (state) {
-            return {state: state.ctx.local, mode: state.ctx.mode};
+            return state.tmp ? {state: state.tmp.local, mode: state.tmp.mode}
+                 : state.ctx ? {state: state.ctx.local, mode: state.ctx.mode}
+                             : null;
         },
 
         token: function (stream, state) {