randr: ProcRRGetOutputInfo: initialize memory

Running Xephyr under valgrind reveals that we're sending some
uninitialized memory over the wire (particularly, the leftover padding
that comes from rounding extraLen to the next 32-bit multiple).

Solve by calloc()ing the memory instead of malloc()ing (the alternative
would be to memset just the padding, but I'm not sure it's more
convenient.)

Signed-off-by: Giuseppe Bilotta <giuseppe.bilotta@gmail.com>
Reviewed-by: Adam Jackson <ajax@redhat.com>
(cherry picked from commit bb766ef1)