Initialize property to UNDEF on unserialize overwrite

The UNDEF marker here is important to prevent the creation of
a reference to the property currently being overwritten, which
wou

Initialize property to UNDEF on unserialize overwrite

The UNDEF marker here is important to prevent the creation of
a reference to the property currently being overwritten, which
would then leak.

This fixes oss-fuzz 6029559193534464, which was incorrectly
merged into oss-fuzz #30584 (which is reported at
https://github.com/google/oss-fuzz/issues/5211).

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