Fix duplicated spec constant on SPIR-V reflection.

(cherry picked from commit 1af1edf)

servers/rendering/rendering_device_driver.cpp

@@ -261,7 +261,7 @@ Error RenderingDeviceDriver::_reflect_spirv(VectorView<ShaderStageSPIRVData> p_s
 							}
 						}
 
-						if (existing > 0) {
+						if (existing >= 0) {
 							r_reflection.specialization_constants.write[existing].stages.set_flag(stage_flag);
 						} else {
 							r_reflection.specialization_constants.push_back(sconst);