client: Don't cancel a roundtrip when any event is received

Since wl_display_dispatch() returns the number of processed events or -1
on error, only cancel the roundtrip if an -1 is returned.

This also fixes a potential memory corruption bug happening when
wl_display_roundtrip() does an early return and the callback later
writes to the then out of scope stack allocated `done' parameter.

Introduced by 33b7637b.

Signed-off-by: Jonas Ådahl <jadahl@gmail.com>