Fix size check in statext_dependencies_deserialize()

Fix size check in statext_dependencies_deserialize()

The check for the minimum expected bytea size of a MVDependencies object
was using SizeOfItem() for its calculation.  This macro uses the number
of attributes in a single dependency.

This minimum size calculation should be based on MinSizeOfItems(), that
computes the minimum expected size as the header plus the
minimally-sized number of dependency items.

Oversight in d08c44f7a4ec.

Author: Ilia Evdokimov <ilya.evdokimov@tantorlabs.com>
Discussion: https://postgr.es/m/4b8d299d-2505-4c30-bf80-0f697410db35@tantorlabs.com
Backpatch-through: 14