Override XShmCreateImage to return null in tracees.

Some tracees may try to create XShm images without checking
XShmQueryExtension first, and we should force them to bail out
as early as possible, before they try to create SysV shmem
objects that we can't (or don't want to) handle.
Merge pull request #1093 from rocallahan/XShmCreateImage-stub

Override XShmCreateImage to return null in tracees.