Invalid position error: data pointers past 2 GiB are unreachable

Since 4.0.0 the reader supports files larger than 2 GB (#289), but Decoder.decodePointer still
decodes the pointer payload into an int. Any data-section offset >= 2^31 wraps negative, and the
subsequent Buffer.position(...) rejects it.

Affected: 4.0.2, 4.1.0, and current main (all identical on this path).

Symptom

java.lang.IllegalArgumentException: Invalid position: -821410056
	at com.maxmind.db.MultiBuffer.position(MultiBuffer.java:...)
	at com.maxmind.db.Decoder.decode(...)
	at com.maxmind.db.NoCache.get(NoCache.java:17)
	at com.maxmind.db.Decoder.decodePointer(...)

-821410056 + 2^32 = 3473557240, a valid offset inside the real production ~3.4 GB database being read. Every
lookup that resolves to a record past the 2 GiB line fails; everything below it works normally.

Cause

var packed = this.decodeInteger(base, pointerSize);
var pointer = packed + this.pointerBase + POINTER_VALUE_OFFSETS[pointerSize];

decodeInteger returns int, so packed is sign-extended by the implicit widening in the
addition. decodeUint32 in the same class already avoids this by going through decodeLong.

Fix

var packed = Decoder.decodeLong(this.buffer, base, pointerSize);

The 3-arg static form is required, since pointer sizes 1-3 carry a non-zero base
(ctrlByte & 0x7).

Happy to open a PR. It comes with a regression test that needs no large fixture.