regex: fix HIR analysis bug

An alternate can be empty at this point, so we must handle it. We didn't
before because the regex engine actually disallows empty alternates,
however, this code runs before the regex compiler rejects the regex.

grep-regex/src/multi.rs

@@ -109,6 +109,7 @@ pub fn alternation_literals(expr: &Hir) -> Option<Vec<Vec<u8>>> {
     for alt in alts {
         let mut lit = vec![];
         match *alt.kind() {
+            HirKind::Empty => {}
             HirKind::Literal(ref x) => extendlit(x, &mut lit),
             HirKind::Concat(ref exprs) => {
                 for e in exprs {