avopt: Fix searching for constants

Fixes Ticket350
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavutil/opt.c

@@ -575,7 +575,7 @@ const AVOption *av_opt_find(void *obj, const char *name, const char *unit,
     while (o = av_next_option(obj, o)) {
         if (!strcmp(o->name, name) && (o->flags & opt_flags) == opt_flags &&
             ((!unit && o->type != FF_OPT_TYPE_CONST) ||
-             (unit  && o->unit && !strcmp(o->unit, unit))))
+             (unit  && o->type == FF_OPT_TYPE_CONST && o->unit && !strcmp(o->unit, unit))))
             return o;
     }
     return NULL;