Fix check whether file name starts with x or X

If path doens't contains / or \, then slashpos will be 0.
Therefore slashpos + 1 = 1 doesn't point to first symbol.

xmesh.nif
 ^
 slashpos + 1

components/nifbullet/bulletnifloader.cpp

@@ -88,18 +88,13 @@ osg::ref_ptr<Resource::BulletShape> BulletNifLoader::load(const Nif::NIFFilePtr&
     else
     {
         bool autogenerated = hasAutoGeneratedCollision(node);
-        bool isAnimated = false;
 
         // files with the name convention xmodel.nif usually have keyframes stored in a separate file xmodel.kf (see Animation::addAnimSource).
         // assume all nodes in the file will be animated
         std::string filename = nif->getFilename();
-        size_t slashpos = filename.find_last_of("/\\");
-        if (slashpos == std::string::npos)
-            slashpos = 0;
-        if (slashpos+1 < filename.size() && (filename[slashpos+1] == 'x' || filename[slashpos+1] == 'X'))
-        {
-            isAnimated = true;
-        }
+        const std::size_t slashpos = filename.find_last_of("/\\");
+        const std::size_t letterPos = slashpos == std::string::npos ? 0 : slashpos + 1;
+        const bool isAnimated = letterPos < filename.size() && (filename[letterPos] == 'x' || filename[letterPos] == 'X');
 
         handleNode(node, 0, autogenerated, isAnimated, autogenerated);