starmath: SmCaretPos's index must be non-negative

Change-Id: I69a2fc0c9ccaa76a0d93127ac6a1544fe9924605
Reviewed-on: https://gerrit.libreoffice.org/32444Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Takeshi Abe <tabe@fixedpoint.jp>

starmath/inc/caret.hxx

@@ -15,6 +15,7 @@
 
 #include "node.hxx"
 
+#include <cassert>
 #include <memory>
 #include <vector>
 
@@ -24,10 +25,13 @@ struct SmCaretPos{
         : pSelectedNode(selectedNode)
         , nIndex(iIndex)
     {
+        assert(nIndex >= 0);
     }
+
     /** Selected node */
     SmNode* pSelectedNode;
-    /** Index within the selected node
+
+    /** Index (invariant: non-negative) within the selected node
      *
      * 0: Position in front of a node
      * 1: Position after a node or after first char in SmTextNode
@@ -38,6 +42,7 @@ struct SmCaretPos{
     //TODO: Special cases for SmBlankNode is needed
     //TODO: Consider forgetting about the todo above... As it's really unpleasant.
     int nIndex;
+
     /** True, if this is a valid caret position */
     bool IsValid() const { return pSelectedNode != nullptr; }
     bool operator==(const SmCaretPos &pos) const {

starmath/source/caret.cxx

@@ -17,7 +17,7 @@ SmCaretPosGraph::~SmCaretPosGraph() = default;
 SmCaretPosGraphEntry* SmCaretPosGraph::Add(SmCaretPos pos,
                                            SmCaretPosGraphEntry* left)
 {
-    SAL_WARN_IF( pos.nIndex < 0, "starmath", "nIndex shouldn't be -1!" );
+    assert(pos.nIndex >= 0);
     auto entry = o3tl::make_unique<SmCaretPosGraphEntry>(pos, left, nullptr);
     SmCaretPosGraphEntry* e = entry.get();
     //Set Left and Right to point to the entry itself if they are NULL