Fix bug related to OIIO texture system

Wrong #ifdef use in the code. If OSL was not used, this caused
sometimes a crash when loading the scene, because the pointer
m_texture_system was not initialised with NULL(0).

Signed-off-by: Marius Avram <marius.avram1309@gmail.com>
Merge pull request #491 from marius-avram/oiio_ifdef_bug

Fix bug related to OIIO texture system