clippy: accounts-db lints

[brooksprumo]

Problem

There are new nightly clippy lints in the accounts-db crate. Refer to #34626 for more information.

warning: this `let...else` may be rewritten with the `?` operator
  --> accounts-db/src/rolling_bit_field/iterators.rs:36:13
   |
36 | /             let Some(bit) = self.bit_range.next() else {
37 | |                 return None;
38 | |             };
   | |______________^ help: replace it with: `let bit = self.bit_range.next()?;`
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#question_mark
   = note: `#[warn(clippy::question_mark)]` on by default

    Checking solana-rpc-client-api v1.18.0 (/Users/brooks/src/solana/rpc-client-api)
warning: `solana-accounts-db` (lib) generated 1 warning

Summary of Changes

Fix 'em.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (09a94b8) 81.8% compared to head (f6a6d32) 81.8%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #34628   +/-   ##
=======================================
  Coverage    81.8%    81.8%           
=======================================
  Files         824      824           
  Lines      222260   222259    -1     
=======================================
+ Hits       181893   181896    +3     
+ Misses      40367    40363    -4     

[HaoranYi]

lgtm