msm: ipa: Set the logbuf NULL after destroy

Set the ipa3_ctx->logbuf NULL after destroy,or
IPAERR will use the ipa3_ctx->logbuf after free it,
that can make crash.

Change-Id: I470c4a043d426d7620c01fadac1f1cd12ad54b08
Signed-off-by: Veerendra Kumar Kamsali <quic_veekum@quicinc.com>