pg_t::get_split_bits: add assert to silence coverity

CID 751330 (#1 of 1): Overflowed return value (INTEGER_OVERFLOW)
 overflow_sink: Overflowed or truncated value (or a value computed
 from an overflowed or truncated value) "p - 1U" used as return value.

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>