type of kiss_fft_scalar may be float, short, int32 or __m128. It is depend on same preprocessor definitions for FIXED_POINT and USE_SIMD. Check that type of kiss_fft_scalar is short.
Your compiler can help you - do not use C cast:
kiss_fft_scalar* samples = &input[0]; // type of &input[0] must be kiss_fft_scalar* !!!