Fix infinite loop in kiss_fft_next_fast_size for n<=0 (issue #121)

kiss_fft_next_fast_size(0) never terminated because the factorization
loop divides m by 2/3/5 forever when m stays 0. Per maintainer decision,
return 1 for n<=0. Add regression test next_fast_size covering n=0,
n<0 and positive inputs.

kiss_fft.c

@@ -411,6 +411,8 @@ void kiss_fft_cleanup(void)
 
 int kiss_fft_next_fast_size(int n)
 {
+    if (n <= 0) /* nothing to factor; avoid infinite loop for n<=0 */
+        return 1;
     while(1) {
         int m=n;
         while ( (m%2) == 0 ) m/=2;