[PATCH] docbook: fix segfault in docproc.c

Adds a missing exit, if the file that should be parsed couldn't be opened.
Without it crashes with a segfault, cause the filedescriptor is accessed
even if the file could not be opened.

Signed-off-by: Henrik Kretzschmar <henne@nachtwindheim.de>
Acked-by: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>