Fix dup_length handling for short reads

[ehsanestaji]

Fixes #145.

When fastqc.dup_length / --dup_length is longer than a read, the overrepresented sequence module currently calls substring(0, dupLength) and crashes with StringIndexOutOfBoundsException.

This change caps the duplication truncation length at the actual sequence length, so short reads are kept whole instead of failing.

Also adds an integration regression test using the existing 16bp minimal.fastq fixture with fastqc.dup_length=20.

Validation:

• Reproduced the original crash with fastqc.dup_length=20 against test/data/minimal.fastq before the fix.
• Compiled the touched production class and new regression test with Java 11.
• Ran DupLengthTest: 1 test found, 1 succeeded, 0 failed.
• Ran unit test package through JUnit launcher: 8 tests found, 8 succeeded, 0 failed.
• Ran git diff --check.

[s-andrews]

Thanks for submitting this. That fix looks good to me, I'll merge it for the next release.