Fixed cppcheck warning about possible null pointer reference

git-svn-id: http://www.openmesh.org/svnrepo/OpenMesh/trunk@573 fdac6126-5c0c-442c-9429-916003d36597

src/OpenMesh/Core/IO/reader/STLReader.cc

@@ -246,7 +246,8 @@ read_stla(const std::string& _filename, BaseImporter& _bi) const
     }
   }
 
-  fclose(in);
+  if (in)
+    fclose(in);
 
   return true;
 }