panics if profile is not found

[rhysh]

Hello! I opened golang/go#67626 for the Go project to track consuming a fix; my guess is there should be an issue on this project to track creation of a fix.

What version of pprof are you using?

I've checked out 9665fa2 from May 20, currently the latest version of the main branch.

What operating system and processor architecture are you using?

linux/amd64

What did you do?

$ go run . ./missing

What did you expect to see?

$ go run . ./missing
./missing: open ./missing: no such file or directory
pprof: failed to fetch any source profiles
exit status 2

What did you see instead?

$ go run . ./missing
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x8906f7]

goroutine 19 [running]:
github.com/google/pprof/internal/driver.fetch({0x7fffec08a721, 0x9}, 0xffffffffc4653600, 0xffffffffc4653600, {0x7fc040045278, 0xc000080210}, {0xa68860, 0xc0000aa840})
	/home/rhysh/pprof/internal/driver/fetch.go:514 +0x2f7
github.com/google/pprof/internal/driver.grabProfile(0xc0001e28c0, {0x7fffec08a721?, 0x0?}, {0x0?, 0x0?}, {0xa69c80, 0xc000032fd0}, {0x7fc040045278, 0xc000080210}, {0xa68860, ...})
	/home/rhysh/pprof/internal/driver/fetch.go:334 +0x131
github.com/google/pprof/internal/driver.concurrentGrab.func1(0xc000234000)
	/home/rhysh/pprof/internal/driver/fetch.go:211 +0x86
created by github.com/google/pprof/internal/driver.concurrentGrab in goroutine 34
	/home/rhysh/pprof/internal/driver/fetch.go:209 +0x8f
exit status 2

Bisect says " a5a03c7 is the first bad commit".

CC @cuishuang, @aalexand

[cuishuang]

Sorry I introduced this unexpected error.