Cleanup: Compiler warning in GPU kernel compilation

Silence false positive about function not returning.

Pull Request: https://projects.blender.org/blender/blender/pulls/146178

intern/cycles/kernel/integrator/volume_stack.h

@@ -23,6 +23,12 @@ ccl_device_forceinline VolumeStack volume_stack_read(const IntegratorGenericStat
   else {
     return integrator_state_read_volume_stack(state, i);
   }
+
+#  ifdef __KERNEL_GPU__
+  /* Silence false positive warning with some GPU compilers. */
+  VolumeStack stack = {};
+  return stack;
+#  endif
 }
 
 template<const bool shadow, typename IntegratorGenericState>