Check for NULL buffer in help command

[Mrcopytuo]

This PR fixes a potential security vulnerability in query-handler that was cloned from {repo} but did not receive the security patch.

###Details:
Affected Function: qh_echo, qh_help and qg_core in query-handler.c
Original Fix: {original commit}

###What this PR does:
This PR applies the same security patch that was applied to the original repository to eliminate the potential vulnerability in the cloned code.

###References:
{original commit}
{link to original CVE/bug id}

Please review and merge this PR to ensure your repository is protected against this potential vulnerability

[sni]

thanks for bringing this to our attention. But i am having trouble reproducing this:

ex.:

echo -ne "#help\0" | socat unix-connect:./tmp/run/naemon.qh -
  help <name>   show help for handler <name>
  help list     list registered handlers

Were you able to crash naemon somehow?

[sni]

After digging a bit in the code, i'd say naemon is not affected by this issue. Seems like the nagios dudes rewrote their query handler a bit and broke things.