StructStoreShared::revalidate() may leave an inconsistent state

[mertemba]

In this section:
https://github.com/jellysheep/structstore/blob/e118e694314b5face75181ff6d6ea0bd11719687/src/structstore_shared.hpp#L207-L231

If the call to mmap_existing_shm() fails/throws and revalidate() returns early, the struct is left in an inconsistent state (shm_ptr probably points to inaccessible memory; shm_fd is open, but apparently fails checks in mmap_existing_shm()).
To mitigate this, the two functions should update the members only when all checks were successful.

cc @joruof