Be more explicit about the cases

Author: maresb

barcode/codex.py

@@ -228,13 +228,14 @@ def _convert_or_buffer(self, char: str) -> int | None:
                 return code128.C[char]
             if char.isdigit():
                 self._buffer += char
-                if len(self._buffer) == 2:
-                    value = int(self._buffer)
-                    self._buffer = ""
-                    return value
-                return None
-            return None
-        return None
+                if len(self._buffer) == 1:
+                    # Wait for the second digit to group in pairs
+                    return None
+                assert len(self._buffer) == 2
+                value = int(self._buffer)
+                self._buffer = ""
+                return value
+        raise RuntimeError(f"Character {char} could not be converted in charset C.")
 
     def _try_to_optimize(self, encoded):
         if encoded[1] in code128.TO: