fix: fix Font Preview crash if opened manually

programs/src/fontpreview/main.cpp

@@ -345,8 +345,13 @@ extern "C" void _start() {
                 g_preview_data = (uint8_t*)montauk::malloc(size);
                 if (g_preview_data) {
                     montauk::read(fd, g_preview_data, 0, size);
-                    if (stbtt_InitFont(&g_preview_info, g_preview_data,
-                                       stbtt_GetFontOffsetForIndex(g_preview_data, 0))) {
+                    // stbtt_GetFontOffsetForIndex returns -1 for anything that
+                    // isn't a valid font (e.g. a directory or arbitrary file).
+                    // Passing that -1 into stbtt_InitFont makes it read at
+                    // data + (uint32)-1, faulting. Guard against it.
+                    int off = stbtt_GetFontOffsetForIndex(g_preview_data, 0);
+                    if (off >= 0 &&
+                        stbtt_InitFont(&g_preview_info, g_preview_data, off)) {
                         g_load_ok = true;
                     }
                 }