Fix borrow error on pre-NLL compilers
Fix borrow error on pre-NLL compilers

    error[E0506]: cannot assign to `missing_content` because it is borrowed
        --> serde_derive/src/de.rs:1414:9
         |
    1388 |           .filter_map(|(i, variant)| {
         |                       -------------- borrow of `missing_content` occurs here
    ...
    1414 | /         missing_content = quote! {
    1415 | |             match __field {
    1416 | |                 #(#missing_content_arms)*
    1417 | |                 #missing_content_fallthrough
    1418 | |             }
    1419 | |         };
         | |_________^ assignment to borrowed `missing_content` occurs here

    error[E0502]: cannot borrow `missing_content_fallthrough` as immutable because it is also borrowed as mutable
        --> serde_derive/src/de.rs:1414:27
         |
    1388 |           .filter_map(|(i, variant)| {
         |                       -------------- mutable borrow occurs here
    ...
    1404 |                       missing_content_fallthrough = quote!(_ => #missing_content);
         |                       --------------------------- previous borrow occurs due to use of `missing_content_fallthrough` in closure
    ...
    1414 |           missing_content = quote! {
         |  ___________________________^
    1415 | |             match __field {
    1416 | |                 #(#missing_content_arms)*
    1417 | |                 #missing_content_fallthrough
    1418 | |             }
    1419 | |         };
         | |_________^ immutable borrow occurs here
    ...
    1622 |   }
         |   - mutable borrow ends here
         |
         = note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)

serde-rs/serde