This fixes the case where e.g.
struct foo_t {
mp_obj_t x;
uint16_t y;
char buf[];
};
will have `sizeof(struct foo_t)==8`, but `offsetof(struct foo_t, buf)==6`.
When computing the size to allocate for `m_new_obj_var` we need to use
offsetof to avoid over-allocating. This is important especially when it
might cause it to spill over into another GC block.
This work was funded through GitHub Sponsors.
Signed-off-by: Jim Mussared <jim.mussared@gmail.com>
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