Current code:
------------------ [BEGIN]
while (differences < end && p < p_end)
{
const SSHORT l = *differences++;
if (l > 0)
{
if (p + l > p_end)
{
BUGCHECK(177); /* msg 177 applied differences will not fit in record */
}
memcpy(p, differences, l); // <--------- CAN READ AFTER LAST ELEMENT OF differences
------------------ [END]

I offer to rewrite this function

------------------ [BEGIN]
USHORT SQZ_apply_differences(Record* const record, const SCHAR* differences, const SCHAR* const end)
{
/**************************************
 *
 * S Q Z _ a p p l y _ d i f f e r e n c e s
 *
 **************************************
 *
 * Functional description
 * Apply a differences (delta) to a record. Return the length.
 *
 **************************************/
typedef USHORT result_type;

fb_assert(record != NULL);
fb_assert(differences <= end);

if (end - differences > MAX_DIFFERENCES)
{
BUGCHECK(176); /* msg 176 bad difference record */
}

SCHAR* p = reinterpret_cast<SCHAR*>(record->rec_data);

const SCHAR* const p_beg = p;
const SCHAR* const p_end = p_beg + record->rec_length;

while (differences < end)
{
const SSHORT l = *differences;

++differences;

//fb_assert(differences <= end);
fb_assert(p >= p_beg);
fb_assert(p <= p_end);

if (l > 0)
{
if (static_cast<size_t>(p_end - p) < static_cast<size_t>(l))
{
BUGCHECK(177); /* msg 177 applied differences will not fit in record */
}

if (static_cast<size_t>(end - differences) < static_cast<size_t>(l)) //<---- an important test
{
BUGCHECK(177); /* or 176? */
}

memcpy(p, differences, l);
p += l;
differences += l;
}
else
{
if (static_cast<size_t>(p_end - p) < static_cast<size_t>(-l))
{
BUGCHECK(177); /* msg 177 applied differences will not fit in record */
}

p += -l;
}//else
}//while (differences < end)

fb_assert(differences == end);
fb_assert(p >= p_beg);
fb_assert(p <= p_end);

//fb_assert(can_numeric_cast<result_type>(p - p_beg));

const result_type length = static_cast<result_type>(p - p_beg);

fb_assert(length <= record->rec_length);

return length;
}//SQZ_apply_differences
------------------ [END]