panic: Safety check panics on non-var reference heads

[tsandall]

This was caught by the nightly fuzz testing. To reproduce:

$ opa run
OPA 0.24.0-dev (commit b5f4c576-dirty, built at 2020-08-21T16:20:51Z)

Run 'help' to see a list of commands and check for updates.

> f([][0]) { true }
panic: interface conversion: ast.Value is *ast.Array, not ast.Var [recovered]
        panic: interface conversion: ast.Value is *ast.Array, not ast.Var

goroutine 1 [running]:
github.com/open-policy-agent/opa/ast.(*Compiler).compile.func1()
        /home/torin/src/opa/ast/compile.go:881 +0x73
panic(0xbdbc20, 0xc0003c32f0)
        /usr/lib/go-1.13/src/runtime/panic.go:679 +0x1b2
github.com/open-policy-agent/opa/ast.outputVarsForTerms.func1(0xc0004b4ea0, 0xc0004ea300)
        /home/torin/src/opa/ast/compile.go:2481 +0x1db
github.com/open-policy-agent/opa/ast.WalkTerms.func1(0xc6d700, 0xc0004b4ea0, 0xc0005526c0)
        /home/torin/src/opa/ast/visit.go:181 +0x42
github.com/open-policy-agent/opa/ast.(*GenericVisitor).Walk(0xc0004ea588, 0xc6d700, 0xc0004b4ea0)
        /home/torin/src/opa/ast/visit.go:270 +0x5b
github.com/open-policy-agent/opa/ast.(*GenericVisitor).Walk(0xc0004ea588, 0xc9cbc0, 0xc0003783c0)
        /home/torin/src/opa/ast/visit.go:320 +0x195
github.com/open-policy-agent/opa/ast.WalkTerms(0xc9cbc0, 0xc0003783c0, 0xc0004ea5d0)
        /home/torin/src/opa/ast/visit.go:185 +0x70

The problem seems to be that these references are not being rewritten.