rust: fix warnings found by nightly compiler

[catenacyber]

Link to redmine ticket:
None

Describe changes:

• Fixes a warning found by rust nightly compiler

My compiler is rustc 1.50.0-nightly (603ab5bd6 2020-11-15)

The full warning text is

warning: getting the inner pointer of a temporary `CString`
  --> src/conf.rs:38:47
   |
38 |         if ConfGet(CString::new(key).unwrap().as_ptr(), &mut vptr) != 1 {
   |                    -------------------------- ^^^^^^ this pointer will be invalid
   |                    |
   |                    this `CString` is deallocated at the end of the statement, bind it to a variable to extend its lifetime
   |
   = note: `#[warn(temporary_cstring_as_ptr)]` on by default
   = note: pointers do not have a lifetime; when calling `as_ptr` the `CString` will be deallocated at the end of the statement because nothing is referencing it as far as the type system is concerned
   = help: for more information, see https://doc.rust-lang.org/reference/destructors.html

[jasonish]

Ok, Important to get this in I think. We might want to backport it as well.

I'm surprised, in other cases with a temporary value, Rustc plain just fails out.

[catenacyber]

https://redmine.openinfosecfoundation.org/issues/4177

[victorjulien]

Merged in #5600, thanks!