rtmpproto: remove dead code

Expression already evaluated before, redundant since
05338686.

Bug-Id: CID 732199

libavformat/rtmpproto.c

@@ -276,9 +276,6 @@ static int rtmp_write_amf_data(URLContext *s, char *param, uint8_t **p)
         *value = '\0';
         value++;
 
-        if (!field || !value)
-            goto fail;
-
         ff_amf_write_field_name(p, field);
     } else {
         goto fail;