Error checking on creating UnitCell pointer

pycrystfel/bindings.pyx

@@ -144,6 +144,8 @@ cdef class UnitCell:
         cdef crystfel.UnitCell* ptr = crystfel.cell_new_from_parameters(
             a, b, c, alpha, beta, gamma,
         )
+        if ptr is NULL:
+            raise MemoryError("Could not create UnitCell struct")
         return UnitCell.from_ptr(ptr, owner=True)
 
     @property