Close fds on pipe() error in tor_spawn_background().

changes/bug4296

@@ -0,0 +1,3 @@
+  o Minor bugfixes:
+    - On a failed pipe() call, don't leak fds. Fix for bug 4296; bugfix
+      on 0.2.3.1-alpha.

src/common/util.c

@@ -3470,6 +3470,10 @@ tor_spawn_background(const char *const filename, const char **argv,
     log_warn(LD_GENERAL,
       "Failed to set up pipe for stderr communication with child process: %s",
       strerror(errno));
+
+    close(stdout_pipe[0]);
+    close(stdout_pipe[1]);
+
     return status;
   }