Fix a simple typo when building with debug turned on.

http://bugs.gentoo.org/show_bug.cgi?id=86339

--- src/chars.c
+++ src/chars.c
@@ -455,7 +455,7 @@
 {
     size_t pos_prev = pos;
 
-    assert(str != NULL && pos <= strlen(buf));
+    assert(buf != NULL && pos <= strlen(buf));
 
     /* There is no library function to move backward one multibyte
      * character.  Here is the naive, O(pos) way to do it. */