When using the rdp backend and connecting with xfreerdp ... --rfx, wlroots
crashes in backend/rdp/output.c while attempting to realloc(..., 0).
This commit guards against that and instead returns true, resulting in
no rfx message being sent. This prevents the crash and appears to work, but
it's not obvious if this is correct from a specification perspective.
a68c7c0c8d