system/FileDescriptor: use std::exchange()

src/system/FileDescriptor.hxx

@@ -33,6 +33,8 @@
 #include "check.h"
 #include "Compiler.h"
 
+#include <utility>
+
 #include <assert.h>
 #include <unistd.h>
 #include <sys/types.h>
@@ -75,11 +77,7 @@ public:
 	}
 
 	int Steal() noexcept {
-		assert(IsDefined());
-
-		int _fd = fd;
-		fd = -1;
-		return _fd;
+		return std::exchange(fd, -1);
 	}
 
 	void SetUndefined() noexcept {