starmath: SmAlignNode has only one child

So, do not bother to extend its children array.

Change-Id: I9550501469aec5c4420f127ab64003c5ab14f9c1
Reviewed-on: https://gerrit.libreoffice.org/19089Reviewed-by: Noel Grandin <noelgrandin@gmail.com>
Tested-by: Noel Grandin <noelgrandin@gmail.com>

starmath/inc/node.hxx

@@ -1178,6 +1178,8 @@ public:
 
 
 /** Node used for alignment
+ *
+ * This node has exactly one child at index 0.
  */
 class SmAlignNode : public SmStructureNode
 {

starmath/source/parse.cxx

@@ -1010,7 +1010,7 @@ void SmParser::DoAlign()
 
     if (pSNode)
     {
-        pSNode->SetSubNodes(popOrZero(m_aNodeStack), 0);
+        pSNode->SetSubNode(0, popOrZero(m_aNodeStack));
         m_aNodeStack.push_front(pSNode);
     }
 }