Remove a misleading line from xmlCharEncOutput

Closes: https://bugzilla.gnome.org/show_bug.cgi?id=793028

It seams this line was accidentally copied over from xmlCharEncOutFunc.
In xmlCharEncOutput output is a pointer so incrementing it by ret can
point it where it wasn't supposed to be pointing. Luckily the current
implementation doesn't dereference the pointer after advancing it.

Signed-off-by: Daniel Veillard <veillard@redhat.com>

encoding.c

@@ -2460,8 +2460,6 @@ retry:
         ret = -3;
     }
 
-    if (ret >= 0) output += ret;
-
     /*
      * Attempt to handle error cases
      */