Fix for CVE-2012-2763 for GIMP 2.6.x by mancha. Based on commit
76155d79df8d497. Thanks to muks, Kevin, and Ankh for identifying the
relevant code change.

Ref: Fixed potential buffer overflow in readstr_upto().

----------

--- a/plug-ins/script-fu/tinyscheme/scheme.c            2012-06-30
+++ b/plug-ins/script-fu/tinyscheme/scheme.c            2012-06-30
@@ -1727,7 +1727,8 @@ static char *readstr_upto(scheme *sc, ch
     c = inchar(sc);
     len = g_unichar_to_utf8(c, p);
     p += len;
-  } while (c && !is_one_of(delim, c));
+  } while ((p - sc->strbuff < sizeof(sc->strbuff)) &&
+          (c && !is_one_of(delim, c)));

   if(p==sc->strbuff+2 && c_prev=='\\')
     *p = '\0';

