CDRIVER-5536 zero-init bson_iter_t::value

[kevinAlbs]

Summary

Zero-initialize bson_iter_t::value.

This PR is related to, but does not resolve CDRIVER-5536.

Background & Motivation

There are many Coverity issues in the libmongocrypt project referencing uninitialized access of the bson_iter_t::value field:

bson_iter_t iter1;
BSON_ASSERT (bson_iter_init (&iter1, &bson)); // Does not initialize `iter1.value`
bson_iter_t iter2 = iter1; // Assigns uninitialized `value`.

I expect this is OK in practice. I do not expect bson_iter_t::value is meant to be directly accessed. The bson_iter_t::value is initialized and returned in bson_iter_value. Regardless, this PR errs towards safety. A caller may reasonably expect bson_iter_init to initialize all struct fields.

[vector-of-bool]

LGTM