Invalid infix causes panic

[kaidesu]

When running the following:

1 + * 2

A panic is thrown by the evaluator:

Ghost (1.0-beta.1)
Press Ctrl + C to exit

>> 1 + * 2
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x28 pc=0x42db55b]

goroutine 1 [running]:
ghostlang.org/x/ghost/evaluator.evaluateInfix(0xc000154120, 0xc0001406a0)
	/Users/kai/Code/Ghost/ghost/src/evaluator/infix.go:24 +0x1bb
ghostlang.org/x/ghost/evaluator.Evaluate({0x430d1a0, 0xc000154120}, 0x43e80f0)
	/Users/kai/Code/Ghost/ghost/src/evaluator/evaluator.go:54 +0x617
ghostlang.org/x/ghost/evaluator.Evaluate({0x430cf20, 0xc000140620}, 0xc00016a480)
	/Users/kai/Code/Ghost/ghost/src/evaluator/evaluator.go:36 +0x4c9
ghostlang.org/x/ghost/evaluator.evaluateProgram(0x4318480, 0x43e80f0)
	/Users/kai/Code/Ghost/ghost/src/evaluator/program.go:12 +0x77
ghostlang.org/x/ghost/evaluator.Evaluate({0x430d360, 0xc0001205e8}, 0x40cfec5)
	/Users/kai/Code/Ghost/ghost/src/evaluator/evaluator.go:18 +0xc8
ghostlang.org/x/ghost/ghost.(*Ghost).Execute(0xc00013dd90)
	/Users/kai/Code/Ghost/ghost/src/ghost/ghost.go:76 +0x18a
ghostlang.org/x/ghost/repl.evaluate(0xc00013dd90, {0xc00012c370, 0x7})
	/Users/kai/Code/Ghost/ghost/src/repl/repl.go:59 +0xb9
ghostlang.org/x/ghost/repl.Start({0x43ebd60, 0xc000128008}, {0x437eabd, 0x18})
	/Users/kai/Code/Ghost/ghost/src/repl/repl.go:45 +0x2e6
main.main()
	/Users/kai/Code/Ghost/ghost/src/cmd/ghost.go:64 +0x677
exit status 2
make: *** [run] Error 1

This seems to be because in the main evaluator loop, we're expecting it to pick up on any ast node that comes through. In the event that we don't find a match, the closing statement is simply returning nil. We should instead throw a runtime error so it can be properly caught and reported back to the end user.

It's fixed already, you can close this now.