fix lint clippy::significant_drop_tightening

[vdbe]

Example of lint for HostHooks

$ RUSTFLAGS="-Zmacro-backtrace" cargo clippy
...
warning: temporary with significant `Drop` can be early dropped
   --> /.../libnss-0.9.0/src/host.rs:139:25
    |
109 |   macro_rules! libnss_host_hooks {
    |   ------------------------------ in this expansion of `libnss_host_hooks!`
...
138 |               extern "C" fn [<_nss_ $mod_ident _sethostent>]() -> c_int {
    |  _______________________________________________________________________-
139 | |                 let mut iter: MutexGuard<Iterator<Host>> = [<HOST_ $mod_ident _ITERATOR>].lock().unwrap();
    | |                         ^^^^
140 | |                 let status = match(<super::$hooks_ident as HostHooks>::get_all_entries()) {
141 | |                     Response::Success(entries) => iter.open(entries),
...   |
144 | |                 status as c_int
145 | |             }
    | |_____________- temporary `iter` is currently being dropped at the end of its contained scope
    |
   ::: src/lib.rs:14:1
    |
14  |   libnss_host_hooks!(custom_hosts, CustomHosts);
    |   --------------------------------------------- in this macro invocation
    |
    = note: this might lead to unnecessary resource contention
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#significant_drop_tightening
    = note: `-W clippy::significant-drop-tightening` implied by `-W clippy::nursery`
    = help: to override `-W clippy::nursery` add `#[allow(clippy::significant_drop_tightening)]`

warning: temporary with significant `Drop` can be early dropped
   --> /.../libnss-0.9.0/src/host.rs:156:25
    |
109 |   macro_rules! libnss_host_hooks {
    |   ------------------------------ in this expansion of `libnss_host_hooks!`
...
155 |                                                                     errnop: *mut c_int) -> c_int {
    |  ________________________________________________________________________________________________-
156 | |                 let mut iter: MutexGuard<Iterator<Host>> = [<HOST_ $mod_ident _ITERATOR>].lock().unwrap();
    | |                         ^^^^
157 | |                 let code: c_int = iter.next().to_c(result, buf, buflen, errnop) as c_int;
158 | |                 if code == NssStatus::TryAgain as c_int {
...   |
161 | |                 return code;
162 | |             }
    | |_____________- temporary `iter` is currently being dropped at the end of its contained scope
    |
   ::: src/lib.rs:14:1
    |
14  |   libnss_host_hooks!(custom_hosts, CustomHosts);
    |   --------------------------------------------- in this macro invocation
    |
    = note: this might lead to unnecessary resource contention
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#significant_drop_tightening
...