Fixed cppcheck warning

src/OpenMesh/Apps/VDProgMesh/mkbalancedpm/mkbalancedpm.cc

@@ -178,9 +178,18 @@ replace_extension( std::string& _s, const std::string& _e )
 {
   std::string::size_type dot = _s.rfind(".");
   if (dot == std::string::npos)
-  { _s += "." + _e; }
+  {
+     // The name does not include a dot and therefore no extension. Adding both.
+     _s += "." + _e;
+  }
   else
-  { _s = _s.substr(0,dot+1)+_e; }
+  {
+     // get everything of the name including the dot but remove the extension.
+     const std::string name = _s.substr(0,dot+1);
+
+     // Add the new extenion
+     _s = name + _e;
+  }
   return _s;
 }