Resolve "Follow up to #2326; Scale_sig32 potentially used on unintialized values in external renderer"

• Related issues: #2341
• Requested reviewers: @pihlajakuja, @vaclav

Reason why this change is needed

• See original issue

Description of the change

• Only call Scale_sig32() on initialized samples.

Affected operating points

• BE, external renderer

Closes #2351 (closed)