bpo-35091: Objects/listobject.c: Replace overflow checks in gallop fu… (GH-10202)

bpo-35091: Objects/listobject.c: Replace overflow checks in gallop fu… (GH-10202)

…nctions with asserts

The actual overflow can never happen because of the following:
* The size of a list can't be greater than PY_SSIZE_T_MAX / sizeof(PyObject*).
* The size of a pointer on all supported plaftorms is at least 4 bytes.
* ofs is positive and less than the list size at the beginning of each iteration.

https://bugs.python.org/issue35091