message: fix(runtime-c-api) `wasmer_last_error_message` returns the length + 1.
  Returning the error message's length + 1 mimics the
  `wasmer_last_error_length` function that counts the trailing null
  byte.
diff: @@ -85,7 +85,7 @@ pub unsafe extern "C" fn wasmer_last_error_message(buffer: *mut c_char, length:
  // accidentally read into garbage.
  buffer[error_message.len()] = 0;
  - error_message.len() as c_int
  + error_message.len() as c_int + 1
  }
  #[derive(Debug)]
  