Fix use of "clever" mktime

Commit 1eef765 introduced regression (conditional _always_ evaluates
to true) in which erlang:localtime_to_universaltime/2 stopped working
on systems configured with timezone without DST (i.e. UTC) on *BSD
platforms:

1> erlang:localtime_to_universaltime({{2012,1,1},{0,0,0}}, true).
** exception error: bad argument

Signed-off-by: Piotr Sikora <piotr.sikora@frickle.com>

erts/emulator/beam/erl_time_sup.c

@@ -757,7 +757,7 @@ local_to_univ(Sint *year, Sint *month, Sint *day,
 	       refuses to give us a DST time, we simulate the Linux/Solaris
 	       behaviour of giving the same data as if is_dst was not set. */
 	    t.tm_isdst = 0;
-	    if (erl_mktime(&the_clock, &t)) {
+	    if (erl_mktime(&the_clock, &t) < 0) {
 		/* Failed anyway, something else is bad - will be a badarg */
 		return 0;
 	    }