Refactor `push_byte`'s code

Move logic below error checking with early exit.

src/link/mod.rs

@@ -100,7 +100,6 @@ impl MessageReader {
                 self.read_bytes += 1;
                 if self.read_bytes >= UE_LENGTH_FIELD_SIZE {
                     self.current_message_length = array_of_u8_to_u32(self.length_buffer) as usize;
-                    self.change_state(ReadingState::Payload);
                     if self.current_message_length > MAX_UE_MESSAGE_LENGTH {
                         self.is_broken = true;
                         return Err(ReadingStreamError::MessageTooLong {
@@ -108,6 +107,7 @@ impl MessageReader {
                         });
                     }
                     self.current_message = Vec::with_capacity(self.current_message_length);
+                    self.change_state(ReadingState::Payload);
                 }
             }
             ReadingState::Payload => {