gstlal_whiten.c: check for fftplans before calling destroy
This PR fixes an issue in the lal_whiten
element where XLALDestroyREAL8FFTPlan was called on FFT plans even if they have not been created, leading to this issue when using LAL with the MKLFFT feature enabled:
XLAL Error - XLALDestroyREAL8FFTPlan (/home/conda/feedstock_root/build_artifacts/lal-split_1620115953393/work/lib/fft/IntelRealFFT_source.c:109): Invalid pointer
XLAL Error - XLALDestroyREAL8FFTPlan (/home/conda/feedstock_root/build_artifacts/lal-split_1620115953393/work/lib/fft/IntelRealFFT_source.c:109): Invalid pointer
Edited by Patrick Godwin