set_gamma_info: remove unnecessary round-trip to server
XRRGetCrtcGamma()
returns the exact same size information as
XRRGetCrtcGammaSize()
, so just use it instead of waiting for
the X server to send it an extra time.
(And if something has gone very wrong so that they aren't the same, this avoids out-of-bounds reads processing the returned gamma arrays by using the size of the arrays we allocated.)
Reported-by: Gregory James DUCK