Fix html (text) at start of phrasing

Closes GH-16.

Author: wooorm

lib/util/container-phrasing.js

@@ -31,7 +31,11 @@ function phrasing(parent, context, safeOptions) {
     // As we can’t escape or so to prevent it from happening, we take a somewhat
     // reasonable approach: replace that eol with a space.
     // See: <https://github.com/syntax-tree/mdast-util-to-markdown/issues/15>
-    if ((before === '\r' || before === '\n') && child.type === 'html') {
+    if (
+      results.length > 0 &&
+      (before === '\r' || before === '\n') &&
+      child.type === 'html'
+    ) {
       results[results.length - 1] = results[results.length - 1].replace(
         /(\r?\n|\r)$/,
         ' '

test.js

@@ -1104,6 +1104,18 @@ test('html', function (t) {
     'should prevent html (text) from becoming html (flow) (3)'
   )
 
+  t.equal(
+    to({
+      type: 'paragraph',
+      children: [
+        {type: 'html', value: '<x>'},
+        {type: 'text', value: 'a'}
+      ]
+    }),
+    '<x>a\n',
+    'should serialize html (text)'
+  )
+
   t.end()
 })