Fix comparing CFIs with different number of !s

Fixes #45

epubcfi.js

@@ -167,7 +167,7 @@ export const compare = (a, b) => {
         || compare(collapse(a, true), collapse(b, true))
 
     for (let i = 0; i < Math.max(a.length, b.length); i++) {
-        const p = a[i], q = b[i]
+        const p = a[i] ?? [], q = b[i] ?? []
         const maxIndex = Math.max(p.length, q.length) - 1
         for (let i = 0; i <= maxIndex; i++) {
             const x = p[i], y = q[i]

tests/epubcfi-tests.js

@@ -227,6 +227,8 @@ const XHTML = str => parser.parseFromString(str, 'application/xhtml+xml')
             '/6/4!/4/12',
             -1,
         ],
+        ['/6/4', '/6/4!/2', -1],
+        ['/6/4!/2', '/6/4!/2!/2', -1],
     ]) {
         const x = CFI.compare(a, b)
         console.assert(x === c, `compare ${a} and ${b}, expected ${c}, got ${x}`)