FmpDevicePkg/FmpDxe: Fix uninitialized pointer dereference

FmpDevicePkg/FmpDxe: Fix uninitialized pointer dereference

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=2602

Zero the allocated buffer in case GetImageInfo `continue` in the middle of
a loop. This will cause unexpected GetImageInfo failure not clearing the
corresponding entry and lead to GP faults when dereferencing this entry.

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