vulkan/wsi/x11: fix shm allocation control flow issue
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shmget returns -1 on error. alloc_shm assigns it to an unsigned variable and then checks whether it's < 0, which will never be true.
Found by Coverity. CID: 1490891
Fixes: 1f55f9a9 ("vulkan/wsi/sw: add support for using host_ptr for shm pixmaps.")