Fix pointer arithmetics in libufdt

According to C standard, it is valid only to calculate pointers to
allocated or one-past-last elements of allocated buffers. Calculating a
pointer to unallocated memory is illegal even if pointer is not
dereferenced.
This change fixes such a pointer calculation.

Fixes: b/379085251
Test: Builds
Test: libufdt tests pass locally
Change-Id: If4787d0135e11f519e1e8346e6c76b880e470199