[folks] PotentialMatch: fix end-of-string check.

[Niels De Graef <nielsdg src gnome org>]

commit 9c4e87e5273f241e45d8bb93369b10903dffeb58
Author: Niels De Graef <nielsdegraef gmail com>
Date:   Fri Jan 5 12:22:33 2018 +0100

    PotentialMatch: fix end-of-string check.
    
    Since we're not dealing with a `\0`-terminated string anymore, we can't
    rely on that to check the end of the string. Add a comparison with the
    length instead.
    
    Fixes bug 7922238 (https://bugzilla.gnome.org/show_bug.cgi?id=792238).

 folks/potential-match.vala |    3 ++-
 1 files changed, 2 insertions(+), 1 deletions(-)
---
diff --git a/folks/potential-match.vala b/folks/potential-match.vala
index f30f743..437381d 100644
--- a/folks/potential-match.vala
+++ b/folks/potential-match.vala
@@ -671,7 +671,8 @@ public class Folks.PotentialMatch : Object
       uint idx = ((int) pos - (int) max_dist).clamp (0, haystack_len - 1);
       unichar ch = 0;
 
-      while (idx < pos + max_dist && (ch = haystack[idx]) != 0)
+      while (idx < pos + max_dist && idx < haystack_len &&
+          (ch = haystack[idx]) != 0)
         {
           if (ch == c)
             return ((int) pos - (int) idx).abs ();