Fix panic when a reference has unspecified lifetime

This will fail later in compilation anyway, but serde_derive needs to
not crash before then.

    #[derive(Deserialize)]
    struct A {
        field: &str,
    }

    error[E0106]: missing lifetime specifier
      --> src/main.rs
       |
       |     field: &str,
       |            ^ expected lifetime parameter

serde_derive_internals/src/attr.rs

@@ -936,7 +936,7 @@ impl Field {
         } else if is_rptr(&field.ty, is_str) || is_rptr(&field.ty, is_slice_u8) {
             // Types &str and &[u8] are always implicitly borrowed. No need for
             // a #[serde(borrow)].
-            borrowed_lifetimes = borrowable_lifetimes(cx, &ident, &field.ty).unwrap();
+            collect_lifetimes(&field.ty, &mut borrowed_lifetimes);
         }
 
         let ser_name = ser_name.get();