rtmp_write_amf_data: remove dead code

Fixes CID732199
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/rtmpproto.c

@@ -256,9 +256,6 @@ 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;