cgi-bin/help-index.c: Check node->section before using it in strcmp()

[zdohnal]

Fixes a segfault in help.cgi, which happens if user tries to search in man pages.
Fedora bug #1921881

[zdohnal]

Reproducer:

1. go to localhost:631 when cups is configured to have web ui and the service is running
2. go to 'Help' tab
3. go to 'Man Pages'
4. insert any text into 'Search' box and click on 'Search'

Result:
help.cgi segfaults, because it tries to compare section and node->section in strcmp(), but node->section is NULL.

Here is the backtrace and printing out node->section.

I'm not sure why node->section here is NULL (it is the node for accounting.html). The nodes are loaded from help.index but it contains a binary data, so I wasn't able to read it.