libafl_nyx: fix stdout cursor position after set_len() (AFLplusplus#3481)

* fix cursor update

* FMT

Author: tokatoka

crates/libafl_nyx/src/executor.rs

@@ -72,6 +72,7 @@ where
             .nyx_stdout
             .set_len(0)
             .map_err(|e| Error::illegal_state(format!("Failed to clear Nyx stdout: {e}")))?;
+        self.helper.nyx_stdout.rewind()?;
 
         let size = u32::try_from(buffer.len())
             .map_err(|_| Error::unsupported("Inputs larger than 4GB are not supported"))?;