runtime/pprof: fix pcDeck's frame indexing

When building the inlining deck, correctly identify which is the last
frame in the deck. Otherwise, when some forms of inlining cause a PC to
expand to multiple frames, the length of the deck's two slices will
diverge.

Fixes #51567

Change-Id: I24e7ba32cb16b167f4307178b3f03c29e5362c4b
Reviewed-on: https://go-review.googlesource.com/c/go/+/391134
Reviewed-by: Michael Pratt <[email protected]>
Trust: Than McIntosh <[email protected]>

Author: rhysh

src/runtime/pprof/proto.go

@@ -530,7 +530,7 @@ func (d *pcDeck) reset() {
 // since the stack trace is already fully expanded) and the symbolizeResult
 // to the deck. If it fails the caller needs to flush the deck and retry.
 func (d *pcDeck) tryAdd(pc uintptr, frames []runtime.Frame, symbolizeResult symbolizeFlag) (success bool) {
-	if existing := len(d.pcs); existing > 0 {
+	if existing := len(d.frames); existing > 0 {
 		// 'd.frames' are all expanded from one 'pc' and represent all
 		// inlined functions so we check only the last one.
 		newFrame := frames[0]