cmd/objdump: guard against out-of-range lines from directives.

//line bogo.go:9999999 will cause 'go tool objdump' to crash
unless bogo.go has that many lines.  Guard the array index
and return innocuous values (nil, nil) from the file cache.

Fixes #36683

Change-Id: I4a9f8444dc611654d270cc876e8848dfd2f84770
Reviewed-on: https://go-review.googlesource.com/c/go/+/223081


Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>