Fix an error with Remarkup when a header-indicator row has more cells…

… than the one above it

Summary:
From fishing through error logs in PHI184. When row 1 has less cells than row 2, and row 2 has a `|--|` section (indicating that the cell above should be a table header), we try to convert an invalid cell into a table header. PHP obliges and creates this cell, but it doesn't have 'content', which produces this error:

> ERROR 8: Undefined index: content at [/core/lib/libphutil/src/markup/engine/remarkup/blockrule/PhutilRemarkupBlockRule.php:160]

Test Plan: Added a failing (well, complaining) test; made it pass.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D18751

src/markup/engine/__tests__/remarkup/table-with-long-header.txt

@@ -0,0 +1,8 @@
+|x|
+||--
+~~~~~~~~~~
+<div class="remarkup-table-wrap"><table class="remarkup-table">
+<tr><td>x</td></tr>
+</table></div>
+~~~~~~~~~~
+| x |

src/markup/engine/remarkup/blockrule/PhutilRemarkupSimpleTableBlockRule.php

@@ -73,7 +73,14 @@ public function markupText($text, $children) {
         // Mark previous row with headings.
         foreach ($cells as $i => $cell) {
           if ($cell['content']) {
-            $rows[last_key($rows)]['content'][$i]['type'] = 'th';
+            $last_key = last_key($rows);
+            if (!isset($rows[$last_key]['content'][$i])) {
+              // If this row has more cells than the previous row, there may
+              // not be a cell above this one to turn into a <th />.
+              continue;
+            }
+
+            $rows[$last_key]['content'][$i]['type'] = 'th';
           }
         }
       }