fix panic: divide by zero

[simakvladimir]

Hi, thx for the great tool!

Problem

In production we use fgprof + alloy + pyroscope. Sometimes we catch panic:

runtime error: integer divide by zero\ngoroutine 11398149 [running]:\nnet/http.(*conn).serve.func1()\n\t/usr/local/go/src/net/http/server.go:1854 +0xbf\npanic({0x15b3320, 0x260a9b0})\n\t/usr/local/go/src/runtime/panic.go:890 +0x263\ngithub.com/felixge/fgprof.(*wallclockProfile).exportPprof(0xc013442a00, 0x0, {0xc000100000?, 0xc015015020?, 0x262e600?}, {0xc005ccd540?, 0x40e007?, 0x262e600?})\n\t/go/src/app/vendor/github.com/felixge/fgprof/fgprof.go:220

Same problem described here.

Panic reproduced in tests when stop called straight after start without sleep.

Possible solution

Check pprof sampling rate on zero before exportPprof calling.

[simakvladimir]

@felixge sorry to interrupt, but could you review PR, please?

[chif84]

Same error. Pls fix.