Fix g_strerror() on non-glibc

When using one of the codepaths that copies the error string into buf,

make sure the string gets strdup() afterward.

https://bugzilla.gnome.org/show_bug.cgi?id=758194

Also see https://git.gnome.org/browse/glib/commit/glib/gstrfuncs.c?id=f87e002313d566dcce71a9aba040d22ddb5c1e80

--- glib/gstrfuncs.c.orig	2015-11-06 18:12:04 UTC

+++ glib/gstrfuncs.c

@@ -1293,6 +1293,8 @@ g_strerror (gint errnum)

           if (error)

             g_print ("%s\n", error->message);

         }

+      else if (msg == (const gchar *)buf)

+        msg = g_strdup(buf);

       g_hash_table_insert (errors, GINT_TO_POINTER (errnum), (char *) msg);

     }