projectlist.cpp: check for null before referencing

Fixes Coverity CID 709302: Dereference before null check (REVERSE_INULL)
Dereferencing pointer "this->m_render". [show details]
 736        int width = (int)(height  * m_render->dar());
...
Dereferencing "this->m_render" before a null check.
 749            if (m_render == NULL) {
 750                kDebug() << "*********  ERROR, NULL RENDR";
 751                return;