cmd/link/internal/loadpe: fix xrels search "not found" detection

Fixes findHandlerInXDataAMD64 to handle the return value of sort.Search
when the search fails to find anything. Otherwise, the value may later
be used as an index, causing an out of range error.

Fixes #64200

Change-Id: I4f92e76b3f4d4d5dbe5cbc707f808298c580afe1
Reviewed-on: https://go-review.googlesource.com/c/go/+/543076
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Carlos Amedee <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Run-TryBot: Quim Muntal <[email protected]>
Reviewed-by: Than McIntosh <[email protected]>
Reviewed-by: Quim Muntal <[email protected]>

Author: dagood

src/cmd/link/internal/loadpe/seh.go

@@ -90,16 +90,17 @@ func findHandlerInXDataAMD64(ldr *loader.Loader, xsym sym.LoaderSym, add int64)
 	// unless it is chained, but we will handle this case later.
 	targetOff := add + unwStaticDataSize*(1+int64(codes))
 	xrels := ldr.Relocs(xsym)
-	idx := sort.Search(xrels.Count(), func(i int) bool {
+	xrelsCount := xrels.Count()
+	idx := sort.Search(xrelsCount, func(i int) bool {
 		return int64(xrels.At(i).Off()) >= targetOff
 	})
-	if idx == 0 {
+	if idx == xrelsCount {
 		return 0
 	}
 	if isChained {
 		// The third relocations references the next .xdata entry in the chain, recurse.
 		idx += 2
-		if idx >= xrels.Count() {
+		if idx >= xrelsCount {
 			return 0
 		}
 		r := xrels.At(idx)