Limit bn_mul_mont() usage to sizes less than or equal to 8192 bits.

Limit bn_mul_mont() usage to sizes less than or equal to 8192 bits.

The assembly bn_mul_mont() implementations effectively use alloca() to
allocate space for computation (at up to 8x the input size), without
any limitation. This means that sufficiently large inputs lead to the
stack being blown. Prevent this by using the C based implementation
instead.

Thanks to Jiayi Lin <jlin139 at asu dot edu> for reporting this to us.

ok beck@ tb@