curl_json plugin: zero-initialize struct correctly

curl_json plugin: zero-initialize struct correctly

Seen on FreeBSD:
curl_json.c: In function 'cj_sock_perform':
curl_json.c:864:10: warning: missing initializer for field 'sun_len' of 'struct sockaddr_un' [-Wmissing-field-initializers]
   struct sockaddr_un sa_unix = {};
          ^
In file included from curl_json.c:32:0:
/usr/include/sys/un.h:48:16: note: 'sun_len' declared here
  unsigned char sun_len; /* sockaddr len including null */
                ^