fix: handle zero sample rate in pprof export

[Jvlegod]

Summary

Fix a panic in pprof export when wall-clock profiling is stopped before the first sample is collected.

In that case, sampleCount is zero, so the measured sample rate rounds to 0. Passing that value into exportPprof causes a divide-by-zero panic while calculating the profile
period.

This patch falls back to the configured sampling rate when the measured rate is less than 1.

Reproducer

A short-lived program can trigger this by starting and immediately stopping pprof-format profiling:

package main

import "github.com/felixge/fgprof"

func main() {
    stop := fgprof.Start(io.Discard, fgprof.FormatPprof)
    _ = stop()
}