Panic in G304 when variable references is not defined in the same file as its usage.

[DrHayt]

Summary

G304 will panic and segfault if the variable referenced is NOT contained in the file where the clean call originates.
This is because ident.Obj may be nil if the referenced declaration is in another file. It also may be incorrect.

Steps to reproduce the behavior

Have a function in file A which has a line that looks like:
whoCares, err := filepath.Rel(variableDefinedInAnotherFile, "unimportant")

Have another file B which contains the variable definition for that variable:
var variableDefinedInAnotherFile string

Scan the path with those two files in it.

BOOM, segfault.

gosec version

Introduced in 2.15.0

Go version (output of 'go version')

go version go1.19.5 darwin/arm64

Operating system / Environment

Darwin tiny.local 22.3.0 Darwin Kernel Version 22.3.0: Mon Jan 30 20:39:35 PST 2023; root:xnu-8792.81.3~2/RELEASE_ARM64_T8103 arm64

Expected behavior

1. No segfault
2. Possibly kvetch about

Actual behavior

panic, segfault, sad faces.

This can be seen the G304 test cases added, or by using v2.15.0 to scan:
github.com/DrHayt/sample

[ccojocar]

fixed by #938