internal/runtime/gc/scan: avoid -1 index when cache sizes unavailable

Fixes golang#74984.
Fixes golang#74983.

Change-Id: I011c66c2005bc4d92f1d17f1f8ce88158634f71f
Reviewed-on: https://go-review.googlesource.com/c/go/+/695476
Auto-Submit: Michael Knyszek <[email protected]>
Reviewed-by: Michael Pratt <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

Author: mknyszek

src/internal/runtime/gc/scan/scan_test.go

@@ -100,6 +100,9 @@ func benchmarkCacheSizes(b *testing.B, fn func(b *testing.B, heapPages int)) {
 			fn(b, pages)
 		})
 	}
+	if len(cacheSizes) == 0 {
+		return
+	}
 	ramPages := int(cacheSizes[len(cacheSizes)-1]*3/2) / gc.PageSize
 	b.Run(fmt.Sprintf("cache=ram/pages=%d", ramPages), func(b *testing.B) {
 		fn(b, ramPages)