fix: guard entry reads when a directory is empty

[lepht]

Closes #6. Adopted from upstream LoveRetro/NextUI#723 by @ericreinsmidt.

What this fixes

top->entries->items[top->selected] was loaded unconditionally in both the input and the render path, so an empty directory read an uninitialised pointer out of the capacity-8 allocation Array_new() never clears. The render path then ran assert(entry) over it — an abort in a debug build, not merely a latent read.

This is the fix #6 asked for, in its own words: hoist the count == 0 check above the load in both paths, rather than relying on the downstream guards. Upstream arrived at the same shape independently:

Entry* entry = total>0 ? top->entries->items[top->selected] : NULL;

The BTN_L1/BTN_R1 alpha-jump handlers get the same total>0 treatment, and the assert goes away. The render body is wrapped in if (entry), with an else branch that forces list_show_entry_names so an empty folder renders a usable state instead of a black screen.

Scope check

After this, every read of entries->items[top->selected] in nextui.c sits behind a count or total>0 check — I audited all of them, not just the two upstream touched. The remaining loop-indexed reads are bounded by count and were never at risk.

One thing this does not do: Array_new() still leaves its capacity uninitialised. That is now unreachable rather than fixed, and I've said so in the commit rather than claiming more than the change earns.

Upstream wrote this for the empty-root case (no ROMs, Tools hidden); it covers the ordinary empty Roms/ folder #6 describes by the same route. #1 separately sidesteps the Favorites-emptied-from-inside path by backing out to root — these are complementary, not redundant.

Testing

Cherry-picks clean onto main. nextui.c passes -fsyntax-only against the desktop platform headers; the only warnings are the pre-existing ones catalogued in #7, including the -Wparentheses hit at line 2681 — none introduced here. Not driven under Xvfb and not tested on hardware.

Sequencing note

This lands in nextui.c where #1 and #2 are actively working, and its BTN_L1/BTN_R1 guards sit exactly where #8's letter indicator would go. Worth re-verifying as that stack merges.

Why now

Upstream is mid-transition from GPL-3.0 to PolyForm Noncommercial (LoveRetro/NextUI#765). Their LICENSE is still GPL-3.0 today, so this is adoptable now; code taken after the transition would not be. Original authorship is preserved in the commit trailer.