Cycles: extra OpenCL NULL point check, maybe avoids some crashes. Don't think

this should ever happen in practice but maybe it does anyway.

intern/cycles/util/util_opencl.cpp

@@ -235,6 +235,9 @@ int clLibraryInit()
     __clewEnqueueBarrier                = (PFNCLENQUEUEBARRIER              )CLCC_DYNLIB_IMPORT(module, "clEnqueueBarrier");
     __clewGetExtensionFunctionAddress   = (PFNCLGETEXTENSIONFUNCTIONADDRESS )CLCC_DYNLIB_IMPORT(module, "clGetExtensionFunctionAddress");
 
+	if(__clewGetPlatformIDs == NULL)
+		return 0;
+
     return 1;
 }