rtsp: do not call fwrite() with NULL pointer FILE *

If the default write callback is used and no destination has been set, a
NULL pointer would be passed to fwrite()'s 4th argument.

OSS-fuzz bug https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=3327
(not publicly open yet)

Detected by OSS-fuzz
Closes #1874

lib/rtsp.c

@@ -756,6 +756,15 @@ CURLcode rtp_client_write(struct connectdata *conn, char *ptr, size_t len)
   }
 
   writeit = data->set.fwrite_rtp?data->set.fwrite_rtp:data->set.fwrite_func;
+
+  if(!data->set.fwrite_rtp && !data->set.is_fwrite_set &&
+     !data->set.rtp_out) {
+    /* if no callback is set for either RTP or default, the default function
+       fwrite() is utilized and that can't handle a NULL input */
+    failf(data, "No destination to default data callback!");
+    return CURLE_WRITE_ERROR;
+  }
+
   wrote = writeit(ptr, 1, len, data->set.rtp_out);
 
   if(CURL_WRITEFUNC_PAUSE == wrote) {