cgi: Fix showing query string in Help

cgi: Fix showing query string in Help

If a string was searched on CUPS Web UI help page, garbage was printed
out in search box. It was because text field pointer was freed before
and contained garbage - previously it was variable value which was
allocated, so the string survived `cgCleanVariables(()`, but the text
field is a pointer into form variables which gets cleaned up.

Fix is to use `strdup()` if `cgiGetTextfield()` returns non-NULL
pointer. The binary exits shortly after either way, so memory is taken
care of by OS.