Don't crash when called on an uninitialised chap structure; this
allows us to avoid extra checks within iscsi.c to make sure that we
receive the CHAP_XXX keys in a sensible order.
We don't actually have a stdio.h header file. Our printf() functions are
defined in vsprintf.h. (This may change, since vsprintf.h is a
non-standard name, but for now it's the one to use.)
There should be no need to include vsprintf.h just for DBG() statements,
since include/compiler.h forces it in for a debug build anyway.