video_core/macro_interpreter: Remove assertion within FetchParameter()

We can just use .at(), which essentially does the same thing, but with
less code.

src/video_core/macro_interpreter.cpp

@@ -223,8 +223,7 @@ void MacroInterpreter::ProcessResult(ResultOperation operation, u32 reg, u32 res
 }
 
 u32 MacroInterpreter::FetchParameter() {
-    ASSERT(next_parameter_index < parameters.size());
-    return parameters[next_parameter_index++];
+    return parameters.at(next_parameter_index++);
 }
 
 u32 MacroInterpreter::GetRegister(u32 register_id) const {