panic during compile if else body missing

[tsandall]

The compiler panics during one of the rewriting stages because it seems like the parser is emiting rules with an empty body:

$ docker run -it --rm  openpolicyagent/opa:0.19.2
OPA 0.19.2 (commit 40f9c1f, built at 2020-04-27T22:50:42Z)

Run 'help' to see a list of commands.

> p { true } else = false
panic: runtime error: index out of range [0] with length 0 [recovered]
	panic: runtime error: index out of range [0] with length 0

goroutine 1 [running]:
github.com/open-policy-agent/opa/ast.(*Compiler).compile.func1()
	/src/ast/compile.go:846 +0x73
panic(0xf662e0, 0xc000144ac0)
	/usr/local/go/src/runtime/panic.go:679 +0x1b2
github.com/open-policy-agent/opa/ast.rewriteDeclaredVarsInBody(0xc000204d60, 0xc000204f80, 0xc0002034d0, 0x160aa90, 0x0, 0x0, 0x0, 0x0, 0x0, 0xc00020e901, ...)
	/src/ast/compile.go:3021 +0x5e1
github.com/open-policy-agent/opa/ast.rewriteLocalVars(0xc000204d60, 0xc000204f80, 0xc0002034d0, 0x160aa90, 0x0, 0x0, 0xb9563b, 0xf6c800, 0xf2a2c0, 0x15abb01, ...)
	/src/ast/compile.go:2992 +0x84
github.com/open-policy-agent/opa/ast.(*Compiler).rewriteLocalVars.func1(0xc000147400, 0x203000)
	/src/ast/compile.go:1098 +0x470
github.com/open-policy-agent/opa/ast.WalkRules.func1(0xf576a0, 0xc000147400, 0xc00014c730)
	/src/ast/visit.go:229 +0x56
github.com/open-policy-agent/opa/ast.(*GenericVisitor).Walk(0xc00020eff8, 0xf576a0, 0xc000147400)
	/src/ast/visit.go:270 +0x5b
github.com/open-policy-agent/opa/ast.(*GenericVisitor).Walk(0xc00020eff8, 0xf576a0, 0xc0001473c0)
	/src/ast/visit.go:295 +0xeb0
github.com/open-policy-agent/opa/ast.(*GenericVisitor).Walk(0xc00020eff8, 0xf30a20, 0xc00014c5f0)
	/src/ast/visit.go:281 +0x77a
github.com/open-policy-agent/opa/ast.WalkRules(0xf30a20, 0xc00014c5f0, 0xc0000b5060)
	/src/ast/visit.go:238 +0x70
github.com/open-policy-agent/opa/ast.(*Compiler).rewriteLocalVars(0xc0001ec340)
	/src/ast/compile.go:1043 +0xe2
github.com/open-policy-agent/opa/ast.(*Compiler).runStage(0xc0001ec340, 0xfdf06e, 0x20, 0xc0002064d0)
	/src/ast/compile.go:832 +0x46
github.com/open-policy-agent/opa/ast.(*Compiler).compile(0xc0001ec340)
	/src/ast/compile.go:851 +0x125
github.com/open-policy-agent/opa/ast.(*Compiler).Compile(0xc0001ec340, 0xc0001e7860)
	/src/ast/compile.go:345 +0x2a3
github.com/open-policy-agent/opa/repl.(*REPL).compileRule(0xc0001a6d80, 0x10e8600, 0xc000026050, 0xc000147080, 0x0, 0x0)
	/src/repl/repl.go:704 +0x3a1
github.com/open-policy-agent/opa/repl.(*REPL).evalStatement(0xc0001a6d80, 0x10e8600, 0xc000026050, 0xf576a0, 0xc000147080, 0x1, 0x1)
	/src/repl/repl.go:865 +0xe1
github.com/open-policy-agent/opa/repl.(*REPL).evalBufferOne(0xc0001a6d80, 0x10e8600, 0xc000026050, 0x0, 0x1)
	/src/repl/repl.go:750 +0x28d
github.com/open-policy-agent/opa/repl.(*REPL).OneShot(0xc0001a6d80, 0x10e8600, 0xc000026050, 0xc000144a80, 0x17, 0x0, 0x0)
	/src/repl/repl.go:255 +0x390
github.com/open-policy-agent/opa/repl.(*REPL).Loop(0xc0001a6d80, 0x10e8600, 0xc000026050)
	/src/repl/repl.go:152 +0x33e
github.com/open-policy-agent/opa/runtime.(*Runtime).StartREPL(0xc00018ec60, 0x10e8600, 0xc000026050)
	/src/runtime/runtime.go:377 +0x3dd
github.com/open-policy-agent/opa/cmd.init.8.func1(0xc000184a00, 0x160aa90, 0x0, 0x0)
	/src/cmd/run.go:138 +0x4df
github.com/spf13/cobra.(*Command).execute(0xc000184a00, 0x160aa90, 0x0, 0x0, 0xc000184a00, 0x160aa90)
	/src/vendor/github.com/spf13/cobra/command.go:766 +0x2aa
github.com/spf13/cobra.(*Command).ExecuteC(0x1599960, 0xc000000180, 0xc0000b5f50, 0x72ed1f)
	/src/vendor/github.com/spf13/cobra/command.go:852 +0x2ea
github.com/spf13/cobra.(*Command).Execute(...)
	/src/vendor/github.com/spf13/cobra/command.go:800
main.main()
	/src/main.go:15 +0x31

We should address this at parse-time.