Remove superfluous variables and assignments in input artifact scanner.

Also turn some variables from pointers into references, as they cannot
be null.
[Pointless assignment uncovered by clang static analyzer.]

Change-Id: I7744b84e601ceef50106dbdd6ecc5dda350df063
Reviewed-by: Joerg Bornemann <joerg.bornemann@theqtcompany.com>
Add a QBS_CHECK in InputArtifactScanner.

This suppresses a false positive from clang static analyzer and is also
of documentational value.

Change-Id: Ia60fda9fab1200c82ce70a682f51f13a78aad5ab
Reviewed-by: Joerg Bornemann <joerg.bornemann@theqtcompany.com>
Add check for null pointer in EvaluatorScriptClass.

[Potential access to null pointer after the loop uncovered by clang
static analyzer.]

Change-Id: I8d46980c83d729963ffcb48c3dd5bc72262f4c29
Reviewed-by: Joerg Bornemann <joerg.bornemann@theqtcompany.com>