Grow needs to allocate and might fail midway. It tries to handle this
using "undo" pattern, and restoring old state on error. But this is
exactly what steps into UAF, as, on error, both errdefer and defer are
run, and the old data is freed.
Instead, use a more robust "reservation" pattern, where we first
fallibly resrve all the resources we need, without applying any changes,
and than do the actual change once we are sure that cannot fail.
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