Fix incorrect error in no-heading-content-indent

Fix an error in `no-heading-content-indent` which occurred if
said heading had no content.

Closes wooorm/remark@177.

Author: wooorm

lib/rules/no-heading-content-indent.js

@@ -59,6 +59,7 @@ function noHeadingContentIndent(ast, file, preferred, done) {
         var depth = node.depth;
         var children = node.children;
         var type = style(node, 'atx');
+        var head;
         var initial;
         var final;
         var diff;
@@ -78,7 +79,17 @@ function noHeadingContentIndent(ast, file, preferred, done) {
             }
 
             index = depth + (index - initial.offset);
-            diff = start(children[0]).column - initial.column - 1 - index;
+            head = start(children[0]).column;
+
+            /*
+             * Ignore empty headings.
+             */
+
+            if (!head) {
+                return;
+            }
+
+            diff = head - initial.column - 1 - index;
 
             if (diff) {
                 word = diff > 0 ? 'Remove' : 'Add';

test/fixtures/no-heading-indent-valid.md

@@ -7,3 +7,7 @@ Foo
 
 Bar
 =====
+
+<!-- Empty headers are fine -->
+
+#####