Fix UB in descriptor_set_layout.cpp

[jherico]

Description

Move the scope of an extension structure so that it has the same call as the parent structure to which it may be attached via a pNext chain.

Fixes #986

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[SaschaWillems]

Great catch. I think the reason this never really caused much of a problem is that it only seems to be used in a single sample.

[marty-johnson59]

Merging - 3 approvals