command: check for fork error before closing fd

We should not set *outfd or *errfd if virExecWithHook() failed
because the caller may close these fds.

Bug present since v0.4.5 (commit 60ed1d2a).
(cherry picked from commit 746ff701)

src/util/command.c

@@ -394,6 +394,10 @@ virExecWithHook(const char *const*argv,
     }
 
     if (pid) { /* parent */
+        if (forkRet < 0) {
+            goto cleanup;
+        }
+
         VIR_FORCE_CLOSE(null);
         if (outfd && *outfd == -1) {
             VIR_FORCE_CLOSE(pipeout[1]);
@@ -404,10 +408,6 @@ virExecWithHook(const char *const*argv,
             *errfd = pipeerr[0];
         }
 
-        if (forkRet < 0) {
-            goto cleanup;
-        }
-
         *retpid = pid;
 
         if (binary != argv[0])