XLALSimInspiralWaveformCache: Don't try to use cache when it is a NULL pointer

Signed-off-by: Benjamin Farr <bfarr@u.northwestern.edu>
LALInference: Pull in changes from Ben and Will's Likelihood to use WaveformCache, but use storage in IFOdata structure

Signed-off-by: Benjamin Farr <bfarr@u.northwestern.edu>