InitExt.c: Add bounds checks for extension request, event, & error codes

Fixes CVE-2023-3138: X servers could return values from XQueryExtension
that would cause Xlib to write entries out-of-bounds of the arrays to
store them, though this would only overwrite other parts of the Display
struct, not outside the bounds allocated for that structure.

Reported-by: Gregory James DUCK <gjduck@gmail.com>
Signed-off-by: Alan Coopersmith <alan.coopersmith@oracle.com>