ipmi: fix a use-after-free bug in error handling
18db96dbfd4a09063a0abcefd51fa8d2aeb115d6 introduced a use-after-free bug
in the error handling of the IPMICTL_RECEIVE_MSG ioctl.
Reported by: Coverity (CID 1490456) (via vangyzen)
Differential Revision: https://reviews.freebsd.org/D35605