Avoid undefined behaviour after realloc()

Merged Alexander Richardson requested to merge arichardson/libx11:fix-realloc-ub into master

Adding the offset between the realloc result and the old allocation to update pointers into the new allocation is undefined behaviour: the old pointers are no longer valid after realloc() according to the C standard. While this works on almost all architectures and compilers, it causes problems on architectures that track pointer bounds (e.g. CHERI or Arm's Morello): the value_list pointers will still have the bounds of the previous allocation and therefore any dereference will result in a run-time trap.

I found this due to a crash (dereferencing an invalid capability) while trying to run xev over SSH on a CHERI-RISC-V system. With these two realloc changes, and xorg/proto/xorgproto!41 (merged) I am able to succesfully run xev compiled for CHERI-RISC-V.

Signed-off-by: Alex Richardson Alexander.Richardson@cl.cam.ac.uk

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