fix usan error in waveforma adjustment - compute sign differently

• Related issues: #1037 (closed)
• Requested reviewers: @multrus @fotopoulou

Reason why this change is needed

• Fix undefined-behaviour bug in EVS part of codec

Description of the change

• Computation of sign is done differently so that it is not undefined or implementation-defined behaviour anymore.

Affected operating points

• EVS PLC might be non-BE, in testing it was still bit-exact, but as undefined behaviour is removed, in theory there could be a difference.

Related to #1037 (closed)