Error message "Invalid MIT-MAGIC-COOKIE-1 key" not followed by a newline character

After the X server has shut down, I get:

$ DISPLAY=:0 xterm
Invalid MIT-MAGIC-COOKIE-1 key/usr/bin/xterm: Xt error: Can't open display: :0

The error message "Invalid MIT-MAGIC-COOKIE-1 key" is not followed by a newline character; this breaks parsing of the output (I initially got this error in a make check). Ditto with other applications linked against libX11 (so that I suppose that this is the culprit).

Moreover, the error message should be preceded by the library name, otherwise one needs to guess where it comes from.

I also reported the bug in the Debian BTS: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=914949