os/HashIndex.cc: fix unintentional integer overflow

CID 1232604 (#1 of 1): Unintentional integer overflow (OVERFLOW_BEFORE_WIDEN)
 overflow_before_widen: Potentially overflowing expression
 (unsigned int)abs(this->merge_threshold) * 16U * this->split_multiplier
 with type unsigned int (32 bits, unsigned) is evaluated using 32-bit
 arithmetic before being used in a context which expects an expression of
 type uint64_t const (64 bits, unsigned). To avoid overflow, cast either
 operand to uint64_t const before performing the multiplication.

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