ANDROID: ashmem_rust: Fix off-by-one errors in get_name()

The current logic returns an error if the name of the buffer plus the
NUL terminating byte is exactly ASHMEM_NAME_LEN. This is incorrect, as
names are expected to have a maximum length of ASHMEM_NAME_LEN, which
accounts for the NUL terminating byte

Thus, change the error condition to be true if the length of the buffer
name plus the NUL terminating byte would be too big to fit in an
ASHMEM_NAME_LEN sized region.

Also, the buffer name is copied into indices [0 len - 1], so the NUL
terminating byte should be at index len, not len + 1.

Bug: 411723589
Change-Id: Ie8ecbad7b90848fea529ca7d043489edd952e333
Signed-off-by: Isaac J. Manjarres <isaacmanjarres@google.com>