util: avoid double close of fd

We could end with a double close if we close the fd loop and flush_fd
fails. That would make us goto fail and there we close the fd once
again. This patch sets the fd to the return value for safe_close: -1
A fd with negative value will be ignored by the next call to
safe_close.

CID#996223

src/shared/util.c

@@ -2074,7 +2074,7 @@ int acquire_terminal(
                  * ended our handle will be dead. It's important that
                  * we do this after sleeping, so that we don't enter
                  * an endless loop. */
-                safe_close(fd);
+                fd = safe_close(fd);
         }
 
         safe_close(notify);