Fix error message on unclosed tag after closed tag

Closes GH-12.

Author: wooorm

lib/index.js

@@ -451,7 +451,9 @@ export function mdxJsxFromMarkdown() {
    * @type {OnEnterError}
    */
   function onErrorRightIsTag(closing, open) {
-    const tag = this.data.mdxJsxTag
+    const stack = this.data.mdxJsxTagStack
+    assert(stack, 'expected `mdxJsxTagStack`')
+    const tag = stack[stack.length - 1]
     assert(tag, 'expected `mdxJsxTag`')
     const place = closing ? ' before the end of `' + closing.type + '`' : ''
     const position = closing

test.js

@@ -246,6 +246,18 @@ test('mdxJsxFromMarkdown', async function (t) {
     }
   )
 
+  await t.test(
+    'should crash on unclosed jsx after closed jsx',
+    async function () {
+      assert.throws(function () {
+        fromMarkdown('<a><b></b>', {
+          extensions: [mdxJsx()],
+          mdastExtensions: [mdxJsxFromMarkdown()]
+        })
+      }, /Expected a closing tag for `<a>` \(1:1-1:4\)/)
+    }
+  )
+
   await t.test(
     'should support an attribute expression in text jsx (agnostic)',
     async function () {