FmpDevicePkg/FmpDxe: Fix Clang build error

FmpDevicePkg/FmpDxe: Fix Clang build error

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=2887

The local Private pointer variable in SetTheImage() is initialized
based on the caller provided This pointer argument. The cleanup
label path uses the Private pointer which will not be
initialized if This is NULL.

This change initializes Private to NULL and accounts for Private
potentially being NULL in the cleanup label path.

Cc: Liming Gao <liming.gao@intel.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Guomin Jiang <guomin.jiang@intel.com>
Cc: Wei6 Xu <wei6.xu@intel.com>
Signed-off-by: Michael Kubacki <michael.kubacki@microsoft.com>
Tested-by: Liming Gao <liming.gao@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>
Reviewed-by: Guomin Jiang <guomin.jiang@intel.com>