CDRIVER-5536 zero-init bson_iter_t::value (#1587)

Intended to address repeated warnings from Coverity. Though leaving `value` uninitialized may be safe, it is also a public struct field.

Author: kevinAlbs

src/libbson/src/bson/bson-iter.c

@@ -62,6 +62,7 @@ bson_iter_init (bson_iter_t *iter,  /* OUT */
    iter->d4 = 0;
    iter->next_off = 4;
    iter->err_off = 0;
+   iter->value = (bson_value_t){0};
 
    return true;
 }