Plug memory leak in plist_write_to_stream()

Thanks @beyonik for pointing this out!
diff --git a/src/plist.c b/src/plist.c
index d78f748..19279ed 100644
--- a/src/plist.c
+++ b/src/plist.c
@@ -1692,6 +1692,7 @@
         if (fwrite(output, 1, length, stream) < length) {
             err = PLIST_ERR_IO;
         }
+        free(output);
     }
     return err;
 }