Fix incorrect size check in statext_dependencies_deserialize

Hi hackers,

I noticed an issue in `statext_dependencies_deserialize()`. The sanity
check uses `SizeOfItem` to validate the bytea size, but `SizeOfItem()`
expects the number of attributes in a single dependency, not the number
of dependencies. This means the check is computing the size of one
dependency with ndeps attributes, which is incorrect.

It should use `MinSizeOfItems` instead, which correctly computes the
minimum expected size as the header plus `ndeps` minimally-sized
dependency items.

Notably, the similar function for ndistinct extended statistics
`statext_ndistinct_deserialize()` already uses `MinSizeOfItems`
correctly, which suggests this is a typo rather than an intentional choice.

--
Best regards,
Ilia Evdokimov,
Tantor Labs LLC,
https://tantorlabs.com/