XLALComputePSDandNormSFTPower: reset *multiPSDVector to NULL if not returned

Description

A minor fix for XLALComputePSDandNormSFTPower(); if *multiPSDVector is not being returned, it should zero out the pointer after destroying the MultiPSDVector. This is important for use from SWIG wrapping, which (if *multiPSDVector is non-NULL) will assume that *multiPSDVector is valid pointer, and may try to double-destroy it leading to a seg-fault.

API Changes and Justification

Backwards Compatible Changes

• This change introduces no API changes
• This change adds new API calls

Backwards Incompatible Changes

• This change modifies an existing API
• This change removes an existing API

If any of the Backwards Incompatible check boxes are ticked please provide a justification why this change is necessary and why it needs to be done in a backwards incompatible way.

Review Status

@david-keitel to review