avoid last recursive call
avoid last recursive call

'make test' output:
### testing SNR for  1024 point FFTs
#### DOUBLE
snr_t2f = 295.35
snr_f2t = 308.32
#### FLOAT
snr_t2f = 146.71
snr_f2t = 143.02
#### SHORT
snr_t2f = 54.718
snr_f2t = 24.494

#### timing 10000 x 1024 point FFTs
#### DOUBLE
Elapsed:0:23.05 user:18.95 sys:0.24
#### FLOAT
Elapsed:0:06.45 user:5.17 sys:0.10
#### SHORT
Elapsed:0:05.59 user:4.72 sys:0.06

sf-is-evil/kissfft,pemryan/kissFFT,sf-is-evil/kissfft,pemryan/kissFFT,pemryan/kissFFT,sf-is-evil/kissfft,sf-is-evil/kissfft,pemryan/kissFFT