Fix a few correctness bugs flagged by SpotBugs (static analysis)

[vejtom]

Ran SpotBugs over the code and a handful of CORRECTNESS warnings looked like real bugs. Small fixes below:

• StreamCopy: the 3-arg copyLength() called itself instead of the 4-arg overload, so it recurses forever. Now it delegates to the 4-arg version.
• ByteOp.drawHex: rows was Math.ceil(length / bytesPerRow), but that's integer division so ceil never rounds up. Cast to double first.
• MetaData(...): putChild() was called on parentMetaData even when it's null, which throws an NPE. Guarded the call.
• GZIPDecoder: lookahead[2] was compared to GZIP_MAGIC_TWO (0x8b) without masking, so a signed byte never matches. Added & 0xff — the same file already does this a few lines down.

Build passes locally.

[ato]

The other changes look good to me.

[vejtom]

Thank you @ato, should be OK now!