avutil/opt: Fix parsing regression with constants starting with numbers

Fixes parsing "3dnow"

Found-by: jamrial
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavutil/opt.c

@@ -206,12 +206,14 @@ static int set_string_number(void *obj, void *target_obj, const AVOption *o, con
         }
 
         {
-            const AVOption *o_named;
+            const AVOption *o_named = av_opt_find(target_obj, buf, o->unit, 0, 0);
             int res;
             int ci = 0;
             double const_values[64];
             const char * const_names[64];
-
+            if (o_named && o_named->type == AV_OPT_TYPE_CONST)
+                d = DEFAULT_NUMVAL(o_named);
+            else {
                 if (o->unit) {
                     for (o_named = NULL; o_named = av_opt_next(target_obj, o_named); ) {
                         if (o_named->type == AV_OPT_TYPE_CONST &&
@@ -246,6 +248,7 @@ static int set_string_number(void *obj, void *target_obj, const AVOption *o, con
                     return res;
                 }
             }
+        }
         if (o->type == AV_OPT_TYPE_FLAGS) {
             read_number(o, dst, NULL, NULL, &intnum);
             if      (cmd == '+') d = intnum | (int64_t)d;