Initialize nvexec variant index before construction

[fallintoplace]

Summary

• initialize the variant discriminator to npos before the default constructor calls emplace
• add a regression that constructs a variant in zero-filled storage and checks that the first alternative is not destroyed before construction

Why

variant_t default construction calls emplace, which calls destroy before constructing the first alternative. Since index_ was not initialized yet, holds_alternative read an indeterminate value and could treat raw storage as an active alternative.

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