Fix nilptr crash in usesCall

[trixnz]

In usesCall(), there are circumstances where the ast.Ident doesn't have an Obj instance, but the line found = ok && id.Obj.Decl == sv.id.Obj.Decl assumes that it it will always have an instance.

I couldn't find an obvious pattern that explains when this happens, but it happens often enough in our code base that I figured I'd just add a nilptr check on the expression.

[coveralls]

Pull Request Test Coverage Report for Build 9565818687

Details

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 83.942%

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Totals
Change from base Build 8792199145:	0.0%
Covered Lines:	345
Relevant Lines:	411

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💛 - Coveralls

[jjti]

Thank you thank you!