Removed division by zero in speed benchmarks

Removed division by zero in speed benchmarks

If some library is so fast that it goes below the resolution of the
timer we're using to measure it, the measured duration may be zero, and
then trigger a division by zero when we convert it to a speed in Hz.

This could possibly happen with a very fast library (Libsodium), on a
very fast machine, with a sufficiently low resolution timer.

This patch reworks and simplifies things a bit, and adds an explicit
check. We now print "too fast to be measured" instead of dividing by
zero.