[Minor] Fix invocation

src/controller.c

 	const rspamd_ftok_t *accept_hdr = rspamd_task_get_request_header(task, "Accept");
 
 	if (accept_hdr && rspamd_substring_search(accept_hdr->begin, accept_hdr->len,
-											  "application/msgpack", sizeof("application/msgpack") - 1)) {
+											  "application/msgpack", sizeof("application/msgpack") - 1) != -1) {
 		ctype = "application/msgpack";
 		out_type = UCL_EMIT_MSGPACK;
 	}

src/libserver/protocol.c

 	accept_hdr = rspamd_task_get_request_header(task, "Accept");
 
 	if (accept_hdr && rspamd_substring_search(accept_hdr->begin, accept_hdr->len,
-											  "application/msgpack", sizeof("application/msgpack") - 1)) {
+											  "application/msgpack", sizeof("application/msgpack") - 1) != -1) {
 		ctype = "application/msgpack";
 		out_type = UCL_EMIT_MSGPACK;
 	}

src/rspamd_proxy.c

 	const rspamd_ftok_t *accept_hdr = rspamd_task_get_request_header(task, "Accept");
 
 	if (accept_hdr && rspamd_substring_search(accept_hdr->begin, accept_hdr->len,
-											  "application/msgpack", sizeof("application/msgpack") - 1)) {
+											  "application/msgpack", sizeof("application/msgpack") - 1) != -1) {
 		ctype = "application/msgpack";
 		out_type = UCL_EMIT_MSGPACK;
 	}