Fix incorrect pointer arithmetics am: f13b6dc7b7

Original change: https://android-review.googlesource.com/c/platform/system/libcppbor/+/3412639

Change-Id: Ic768e8f5ea9737fa57d777bfd431e93f01dd6acd
Signed-off-by: Automerger Merge Worker <[email protected]>