internal/profile: potential panic during PGO build due to nil Location in Sample

[nicholashusin]

This was originally reported as a security issue. We do not believe that this is a vulnerability, and are therefore filing it as a regular bug.

Minimal repro in src/internal/profile/profile_test.go:

func TestParseDanglingLocationID(t *testing.T) {
	// Profile containing a sample referencing non-existent location ID 99.
	raw, _ := hex.DecodeString("3200320773616d706c65733205636f756e740a040801100212050a01631001")
	p, err := Parse(bytes.NewReader(raw))
	if err != nil {
		return // Expected failure
	}
	// If Parse accepted it, NewGraph should not panic.
	_ = NewGraph(p, &Nodes{})
}

[prattmic]

cc @golang/compiler

[gopherbot]

Change https://go.dev/cl/828404 mentions this issue: internal/profile: reject dangling location IDs in Parse