avfilter/avf_showcqt: avoid using fminf()

The loop with fminf() changes from 18093856 to 17403218 dezicycles (gcc 4.6.3, sandybridge i7)

Reviewed-by: Clément Bœsch <u@pkh.me>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
