ipmi: test for event buffer before using

The IPMI driver would attempt to use the event buffer even if that
didn't exist on the BMC.  This patch modified the IPMI driver to check
for the event buffer's existence before trying to use it.

Signed-off-by: Corey Minyard <minyard@acm.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>