Fix crash when scrolling to oldest message on an empty table

[fayharinn]

Fixes #291.

performScrollTo's .oldestMessage branch clamps the section index with max(numberOfSections - 1, 0), so an empty table still asks UIKit for section 0:

NSInternalInconsistencyException: Requested the number of rows for section (0) which is out of bounds.
-[UITableViewRowData numberOfRowsInSection:]

The existing guard lastRow >= 0 never fires, because max(_, 0) already forced the value non-negative, and it runs after the offending call anyway.

This guards on the section count first, then drops both clamps so the row guard becomes meaningful for an empty section. No behaviour change on a populated table.

The path is reachable in normal use: onStatusBarTap arms pendingScrollTo = .oldestMessage, and that subscription is a global NotificationCenter observer released in onDisappear, which does not fire while a ChatView stays mounted off screen (a TabView tab, or lower in a NavigationStack). A status bar tap from another screen then scrolls a chat whose table is still empty. Issue #291 has the full trace and reproduction steps.

Observed on iOS 27.0 with 3.1.7; the same code is on main and in 3.2.7.

[f3dm76]

Hey @fayharinn, thank you so much for the fix, have a brilliant day!