Fix panic in BuiltinPager drop when pager thread panics

The Drop impl for OutputType::BuiltinPager calls
handle.take().unwrap().join().unwrap() which panics if the
pager thread itself panicked. In Drop, this causes a double
panic (abort).

Replace with if-let and discard the join result, matching the
pattern used for OutputType::Pager which also uses let _ =.

Reported in #3449 where BAT_PAGER=builtin with --help
causes the pager thread to fail.

Fixes #3449

src/output.rs

@@ -225,8 +225,8 @@ impl Drop for OutputType {
                 let _ = command.wait();
             }
             OutputType::BuiltinPager(ref mut pager) => {
-                if pager.handle.is_some() {
-                    let _ = pager.handle.take().unwrap().join().unwrap();
+                if let Some(handle) = pager.handle.take() {
+                    let _ = handle.join();
                 }
             }
             OutputType::Stdout(_) => (),