unserialize: Strictly check for `:{` at object start (#10214)

* unserialize: Strictly check for `:{` at object start

* unserialize: Update CVE tests

It's unlikely that the

unserialize: Strictly check for `:{` at object start (#10214)

* unserialize: Strictly check for `:{` at object start

* unserialize: Update CVE tests

It's unlikely that the object syntax error contributed to the actual CVE. The
CVE is rather caused by the incorrect object serialization data of the `C`
format. Add a second string without such a syntax error to ensure that path is
still executed as well to ensure the CVE is absent.

* Fix test expectation in gmp/tests/bug74670.phpt

No changes to the input required, because the test actually is intended to
verify the behavior for a missing `}`, it's just that the report position changed.

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