glx: Fix possible out of bounds access in __glXDisp_GetVisualConfigs()

If p is odd, then the zeroing loop will access memory out of bounds by
one element. We can prevent that by zeroing just single element at once.

Signed-off-by: Povilas Kanapickas <povilas@radix.lt>
4 jobs for fix-possible-out-of-bounds-access-glxcmds in 4 minutes and 31 seconds (queued for 1 second)