cgen: fix initialize error object in or_block (fix #24529)

[gchumillas]

Fixes #24529

There's no guarantee that memcpy will work correctly, since err_obj and cvar_name can be of different types. Instead of using memcpy, we declare a fresh variable and initialize its values.

All tests passed on a MacBook Pro M1:

v self
v test-self

[huly-for-github]

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