bessel: check that the function terminates as expected by av_assert2().

A clear abort is better than wrong output and a possible crash.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libswresample/resample.c

@@ -78,6 +78,7 @@ static double bessel(double x){
         lastv=v;
         t *= x*inv[i];
         v += t;
+        av_assert2(i<99);
     }
     return v;
 }