Bugfix: Don’t crash when asprintf() can’t print the window name (Thanks ch3ka)

Bugfix: Don’t crash when asprintf() can’t print the window name (Thanks ch3ka)

Apparantly, under some conditions (using LANG=en_US.UTF-8, other locale
variables unset), asprintf() says "Invalid or incomplete multibyte or
wide character" when given a string in COMPOUND_TEXT encoding.

For now, we properly handle asprintf-errors (this should have been
before), but there might be a better solution.