memory/GrowingBuffer: use std::exchange()

src/memory/GrowingBuffer.hxx

@@ -127,9 +127,7 @@ public:
 	GrowingBuffer() = default;
 
 	GrowingBuffer(GrowingBuffer &&src) noexcept
-		:head(std::move(src.head)), tail(src.tail) {
-		src.tail = nullptr;
-	}
+		:head(std::move(src.head)), tail(std::exchange(src.tail, nullptr)) {}
 
 	GrowingBuffer &operator=(GrowingBuffer &&src) noexcept {
 		using std::swap;