Avoid "Can't substr beyond end of string" errors -- if we're already

beyond the end of the target, then just fail.

src/PAST/Compiler-Regex.pir

@@ -130,6 +130,7 @@ Return the POST representation of the regex AST rooted by C<node>.
     ops.'push_pirop'('.local pmc', 'match')
     ops.'push_pirop'('.lex', '"$/"', 'match')
     ops.'push_pirop'('length', eos, tgt, 'result'=>eos)
+    ops.'push_pirop'('gt', pos, eos, donelabel)
 
     # On Parrot, indexing into variable-width encoded strings
     # (such as utf8) becomes much more expensive as we move