xwayland: Fix out of bounds write

Detected using ASAN, declaration of the type is:
typedef union xcb_client_message_data_t {
    uint8_t  data8[20];
    uint16_t data16[10];
    uint32_t data32[5];
} xcb_client_message_data_t;


(cherry picked from commit 6b1ccb8b)