MakeSFTs.c: fix incorrect XLAL error checks

Description

When porting lalapps_MakeSFTs to use the XLAL FFT error functions in !921 (merged), incorrect XLAL_CHECK() statements were added that check for a non-zero success return from an XLAL function, rather than comparing to XLAL_SUCCESS (i.e. checking for a zero success return). This results in lalapps_MakeSFTs failing for no reason. This error has made it into the installed version on clusters and is preventing SFT production for O3. So this will need a new LALApps point release once merged.

Closes https://git.ligo.org/CW/software/lalsuite/issues/62

@adam-mercer @duncanmmacleod @kipp.cannon @david-keitel

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

Review Status

Checked that lalapps_MakeSFTs now works with this change. (lalapps_MakeSFTs really needs a test suite.)