Fix butterfly optimization segfault on already optimized Faust.

Bugged code:
In [1]: from pyfaust.fact import butterfly

In [2]: from pyfaust import opt_butterfly_faust

In [3]: butterfly?

In [4]:     >>> import numpy as np
   ...:     >>> from random import randint
   ...:     >>> from pyfaust.fact import butterfly
   ...:     >>> from pyfaust import Faust, wht, dft
   ...:     >>> H = wht(8).toarray()
   ...:     >>> F = butterfly(H, type='bbtree')
   ...:

In [5]: F
Out[5]:
Faust size 8x8, density 0.75, nnz_sum 48, 3 factor(s):
- FACTOR 0 (double) SPARSE, size 8x8, density 0.25, nnz 16
- FACTOR 1 (double) SPARSE, size 8x8, density 0.25, nnz 16
- FACTOR 2 (double) SPARSE, size 8x8, density 0.25, nnz 16

In [6]: opt_butterfly_faust(F)
Out[6]:
Faust size 8x8, density 0.75, nnz_sum 48, 3 factor(s):
- FACTOR 0 (double) Butterfly, size 8x8, density 0.25, nnz 16
- FACTOR 1 (double) Butterfly, size 8x8, density 0.25, nnz 16
- FACTOR 2 (double) Butterfly, size 8x8, density 0.25, nnz 16

In [7]: oF = opt_butterfly_faust(F)

In [8]: opt_butterfly_faust(oF)
Erreur de segmentation (core dumped)