dcs: XTGETTCAP: handle empty request

If the XTGETTCAP request is empty (no capabilities in it), reply with
an empty error reply.

Closes #1694

CHANGELOG.md

@@ -62,6 +62,9 @@
 * Crash when zooming in or out, with `dpi-aware=yes`, and the
   monitor's DPI is 0 (this is true for, for example, nested Wayland
   sessions, or in virtualized environments).
+* No error response for empty `XTGETTCAP` request ([#1694][1694]).
+
+[1694]: https://codeberg.org/dnkl/foot/issues/1694
 
 
 ### Security

dcs.c

@@ -200,6 +200,12 @@ xtgettcap_unhook(struct terminal *term)
     const char *const end = (const char *)&term->vt.dcs.data[left];
     const char *p = (const char *)term->vt.dcs.data;
 
+    if (p == NULL) {
+        /* Request is empty; send an error reply, without any capabilities */
+        term_to_slave(term, "\033P0+r\033\\", 7);
+        return;
+    }
+
     while (true) {
         const char *sep = memchr(p, ';', left);
         size_t cap_len;