Allow modification of StreamBuffer (data values, size).

firmware/common/message.hpp

@@ -406,7 +406,7 @@ public:
 		return used_ >= capacity_;
 	}
 
-	const void* data() const {
+	void* data() const {
 		return data_;
 	}
 
@@ -414,6 +414,10 @@ public:
 		return used_;
 	}
 
+	void set_size(const size_t value) {
+		used_ = value;
+	}
+
 	void empty() {
 		used_ = 0;
 	}