Avoid bug when replacement stop at end of string

Detect when we are past the end of the string to avoid by
checking if byte is not nil.

Fix #510.

data/core/regex.lua

@@ -23,7 +23,7 @@ end
 -- Moves to the end of the identified character.
 local function end_character(str, index)
   local byte = string.byte(str, index + 1)
-  while byte >= 128 and byte < 192 do
+  while byte and byte >= 128 and byte < 192 do
     index = index + 1
     byte = string.byte(str, index + 1)
   end