util: Fix bug which will cause libvirtd crash

"virCommandRun": if "cmd->outbuf" or "cmd->errbuf" is NULL,
libvirtd will be crashed when trying to start a qemu domain
(which invokes "virCommandRun"), it caused by we try to use
"*cmd->outbuf" and "*cmd->errbuf" regardless of cmd->outbuf
or cmd->errbuf is NULL.

* src/util/command.c (virCommandRun)

src/util/command.c

@@ -931,8 +931,8 @@ virCommandRun(virCommandPtr cmd, int *exitstatus)
         ret = -1;
 
     VIR_DEBUG("Result stdout: '%s' stderr: '%s'",
-              NULLSTR(*cmd->outbuf),
-              NULLSTR(*cmd->errbuf));
+              cmd->outbuf ? NULLSTR(*cmd->outbuf) : "(null)",
+              cmd->errbuf ? NULLSTR(*cmd->errbuf) : "(null)");
 
     /* Reset any capturing, in case caller runs
      * this identical command again */