debug/elf: check for multiplication overflow for shnum * shentsize

debug/elf: check for multiplication overflow for shnum * shentsize

No test case because the problem can only happen for invalid data.
Let the fuzzer find cases like this.

For #47653
Fixes #70584

Change-Id: I8a69a27dcb5b258b88f8e01ebaf0ec20cfcd489b
Reviewed-on: https://go-review.googlesource.com/c/go/+/632035
Reviewed-by: Veronica Silina <veronicasilina@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Auto-Submit: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>