lavu/opt: Use "&&" instead of "*" in boolean expression.

Fixes the following warning:
libavutil/opt.c:101:47: warning: '*' in boolean context, suggest '&&' instead

libavutil/opt.c

@@ -98,7 +98,7 @@ static int write_number(void *obj, const AVOption *o, void *dst, double num, int
 {
     if (o->type != AV_OPT_TYPE_FLAGS &&
         (!den || o->max * den < num * intnum || o->min * den > num * intnum)) {
-        num = den ? num * intnum / den : (num * intnum ? INFINITY : NAN);
+        num = den ? num * intnum / den : (num && intnum ? INFINITY : NAN);
         av_log(obj, AV_LOG_ERROR, "Value %f for parameter '%s' out of range [%g - %g]\n",
                num, o->name, o->min, o->max);
         return AVERROR(ERANGE);