PYTHON-4663 Fix coverity warnings in datetime decoding change

[ShaneHarvey]

PYTHON-4663

First warning:

1. returned_null: PyModule_GetState returns NULL (checked 18 out of 19 times).
     	2. var_assigned: Assigning: state = NULL return value from PyModule_GetState.
389    struct module_state *state = GETSTATE(self);

Second:

CID 162119: (#2 of 2): Unintentional integer overflow (OVERFLOW_BEFORE_WIDEN)
overflow_before_widen: Potentially overflowing expression ((PyDateTime_Delta *)utcoffset)->days * 86400 with type int (32 bits, signed) is evaluated using 32-bit arithmetic, and then used in a context that expects an expression of type int64_t (64 bits, signed).
     	To avoid overflow, cast either ((PyDateTime_Delta *)utcoffset)->days or 86400 to type int64_t.
417                min_millis_offset = (PyDateTime_DELTA_GET_DAYS(utcoffset) * 86400 +
418                                     PyDateTime_DELTA_GET_SECONDS(utcoffset)) * 1000 +
419                                     (PyDateTime_DELTA_GET_MICROSECONDS(utcoffset) / 1000);

[blink1073]

LGTM!