debug/elf: don't panic if symtab too small

debug/elf: don't panic if symtab too small

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

For #47653
Fixes #75130

Change-Id: Ie6015564bb98334377383bbc16d79119dc4e36f9
Reviewed-on: https://go-review.googlesource.com/c/go/+/698855
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Auto-Submit: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Carlos Amedee <carlos@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Florian Lehner <lehner.florian86@gmail.com>