tools/rados: fix leaked/unflushed output stream in 'ls'

`if (!stdout)` tested the libc FILE* (never null) instead of the
use_stdout flag, so the ofstream was never deleted when writing to
a file, leaking it and leaving its buffer unflushed. Test use_stdout.

Signed-off-by: Sun Yuechi <sunyuechi@iscas.ac.cn>

src/tools/rados/rados.cc

@@ -2581,7 +2581,7 @@ static int rados_tool_common(const std::map < std::string, std::string > &opts,
       }
       formatter->flush(*outstream);
     }
-    if (!stdout) {
+    if (!use_stdout) {
       delete outstream;
     }
   }