Fix: add missing overflow check in bt_ctf_stream_pos_access_ok

Found by Coverity:

2. overflow: Subtract operation overflows on operands bit_offset and 1UL.

CID 1377278 (#1 of 1): Overflowed return value (INTEGER_OVERFLOW).
overflow_sink: Overflowed or truncated value (or a value computed from
an overflowed or truncated value) bt_ctf_stream_pos_move(pos, ({...}))
used as return value.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>