kconfig: exit if no beginning filename

If the beginning Kconfig file is missing, config segfaults so it might as
well exit after the error message.

Signed-off-by: Randy Dunlap <rdunlap@xenotime.net>
Cc: Roman Zippel <zippel@linux-m68k.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>