debug/pe: omit panic in (*File).ImportedSymbols on empty optional headers

If a PE file with invalid optional header size (neither
sizeofOptionalHeader32 nor sizeofOptionalHeader64) is passed to NewFile,
the File.OptionalHeader will be nil which leads to a panic in
(*File).ImportedSymbols().

Fixes #30250

Change-Id: Ie97306de4a0e2dcfdc7b1b599891f574aa63adca
Reviewed-on: https://go-review.googlesource.com/c/162858


Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>