minor fix

Author: NimaSarajpoor

stumpy/sdp.py

@@ -282,4 +282,7 @@ def __call__(self, Q, T, n_threads=1, planning_flag="FFTW_ESTIMATE"):
         return real_arr[m - 1 : n]
 
 
-_pyfftw_sliding_dot_product = _PYFFTW_SLIDING_DOT_PRODUCT()
+if FFTW_IS_AVAILABLE:
+    _pyfftw_sliding_dot_product = _PYFFTW_SLIDING_DOT_PRODUCT()
+else:  # pragma: no cover
+    _pyfftw_sliding_dot_product = None