cmdutils: Fix overriding flags on the command line.

Previously the code just appended the strings of flags which
worked with "+bitexact" but would not work with something like "0"
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

cmdutils.c

@@ -532,7 +532,7 @@ static const AVOption *opt_find(void *obj, const char *name, const char *unit,
     return o;
 }
 
-#define FLAGS (o->type == AV_OPT_TYPE_FLAGS) ? AV_DICT_APPEND : 0
+#define FLAGS (o->type == AV_OPT_TYPE_FLAGS && (arg[0]=='-' || arg[0]=='+')) ? AV_DICT_APPEND : 0
 int opt_default(void *optctx, const char *opt, const char *arg)
 {
     const AVOption *o;