Fix Cycles OpenCL issue if context/program creation fails, mistake by me,

patch #35866 by Doug Gale to fix it.

intern/cycles/device/device_opencl.cpp

@@ -241,6 +241,9 @@ public:
 	{
 		cl_context context = get_something<cl_context>(platform, device, &Slot::context, slot_locker);
 
+		if(!context)
+			return NULL;
+
 		/* caller is going to release it when done with it, so retain it */
 		cl_int ciErr = clRetainContext(context);
 		assert(ciErr == CL_SUCCESS);
@@ -255,6 +258,9 @@ public:
 	{
 		cl_program program = get_something<cl_program>(platform, device, &Slot::program, slot_locker);
 
+		if(!program)
+			return NULL;
+
 		/* caller is going to release it when done with it, so retain it */
 		cl_int ciErr = clRetainProgram(program);
 		assert(ciErr == CL_SUCCESS);