p2p: fix warning for uninitialized variable (clang)

This is actually a false positive only because
p2p_device_validate_conn_wfd bails out if the IE is NULL which
avoids using wfd_data_length. But its subtle and without inspecting
the code it does seem like the length could be used uninitialized.

src/p2p.c:940:7: error: variable 'wfd_data_len' is used uninitialized whenever 'if' condition is false [-Werror,-Wsometimes-uninitialized]
                if (dev->conn_own_wfd)
                    ^~~~~~~~~~~~~~~~~
src/p2p.c:946:8: note: uninitialized use occurs here
                                                        wfd_data_len))
                                                        ^~~~~~~~~~~~
src/p2p.c:940:3: note: remove the 'if' if its condition is always true
                if (dev->conn_own_wfd)
                ^~~~~~~~~~~~~~~~~~~~~~
src/p2p.c:906:23: note: initialize the variable 'wfd_data_len' to silence this warning
                ssize_t wfd_data_len;
                                    ^
                                     = 0

src/p2p.c

@@ -903,7 +903,7 @@ static void p2p_group_event(enum ap_event_type type, const void *event_data,
 	{
 		const struct ap_event_station_added_data *data = event_data;
 		L_AUTO_FREE_VAR(uint8_t *, wfd_data) = NULL;
-		ssize_t wfd_data_len;
+		ssize_t wfd_data_len = 0;
 		struct p2p_association_req req_info;
 		int r;