Fix startup crash from an out-of-range customdirectoryindex

[mcfnord]

🤖 AI: Fixes #3917, which @pljones asked for a PR on. The client accepts a customdirectoryindex of MAX_NUM_SERVER_ADDR_ITEMS out of the settings file and then uses it as a subscript into a vector holding exactly that many elements, so the one value past the end crashes the client at startup. The range check is one wider than the object it guards.

Short description of changes

One line in CClientSettings::ReadSettingsFromXML: the upper bound becomes MAX_NUM_SERVER_ADDR_ITEMS - 1. GetNumericIniSet's bound is inclusive, MAX_NUM_SERVER_ADDR_ITEMS is 12 and vstrDirectoryAddress holds 12 elements, so 12 was accepted and 12 is out of range. The two unguarded subscripts are in CConnectDlg::RequestServerList and the jamulusclient/getCurrentDirectory RPC method, so it is not GUI-only.

The client's own writer only ever emits 0..11, so reaching this needs a hand-edited or corrupted settings file. The range check exists to make that safe.

Both arms, built from the same tree with and without this one line, Qt 5.15.13, gcc 13.3, QT_QPA_PLATFORM=offscreen, settings file carrying directorytype 7 (AT_CUSTOM) and winviscon 1:

customdirectoryindex in the file	main	with this change
11	connect dialog drawn, value kept	connect dialog drawn, value kept
12	SIGSEGV, exit 139, dialog never drawn	connect dialog drawn, value falls back to 0

The fallback is visible in the settings file the client writes back on exit: 0 where 12 was rejected, 11 where 11 was accepted.

CHANGELOG: Client: Fixed a startup crash caused by an out-of-range custom directory index in the settings file.

Context: Fixes an issue?

Fixes: #3917

Does this change need documentation? What needs to be documented and how?

No.

Status of this Pull Request

Ready for review.

What is missing until this pull request can be merged?

Review.

Checklist

• I've verified that this Pull Request follows the general code principles
• I tested my code and it does what I want — evidence above
• My code follows the style guide
• I waited some time after this Pull Request was opened and all GitHub checks completed without errors.
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🤖 This message was written by AI and reviewed by @mcfnord.

[softins]

Looks ok to me. I usually prefer the upper bound of a range check to be exclusive rather than inclusive, but this change matches the convention established by all the other uses of GetNumericIniSet()