fix pop panic when length is 0

[tjblackheart]

Hi,

on rare occasions (not sure why - I cannot really create a working failed state, as this happens at random) the twig autoEscapeVisitor is empty and throws a panic when pop() is called. Looks like this:

http: panic serving 172.17.0.1:42832: runtime error: slice bounds out of range [:-1]
goroutine 10919 [running]:
net/http.(*conn).serve.func1(0xc000443360)
	/usr/local/go/src/net/http/server.go:1804 +0x153
panic(0xa1d120, 0xc00038e978)
	/usr/local/go/src/runtime/panic.go:971 +0x499
github.com/tyler-sommer/stick/twig.(*autoEscapeVisitor).pop(...)
	/go/pkg/mod/github.com/tyler-sommer/stick@v1.0.3/twig/escape.go:73
github.com/tyler-sommer/stick/twig.(*autoEscapeVisitor).Leave(0xc0000a2750, 0xaf2290, 0xc0001cb960)
	/go/pkg/mod/github.com/tyler-sommer/stick@v1.0.3/twig/escape.go:105 +0x7
...

This PR should take care of that.

[tyler-sommer]

It sounds like maybe entering/leaving is unbalanced somewhere but this should be an acceptable workaround. Thanks for the PR!