fix(support_rp2040): do not move the cursor forward if the sink is stuck

Fixes serial output corruption that occurs when a string longer than the
FIFO size is outputted by `sprint!`.

Author: yvt

src/r3_support_rp2040/src/stdout.rs

@@ -51,10 +51,10 @@ impl WrapSerialWrite {
                             [head, tail @ ..] => {
                                 // Output the first byte. If this gets stuck,
                                 // break out of `interrupt::free`.
-                                s = tail;
                                 stdout
                                     .write(*head)
                                     .map_err(|e| e.map(|_| core::fmt::Error))?;
+                                s = tail;
                             }
                         }
                     }