config: error out on empty key- or mouse binding

Closes #851

CHANGELOG.md

@@ -62,6 +62,8 @@
   CSI codes in the Kitty keyboard protocol.
 * Assertion in `shm.c:buffer_release()`
   (https://codeberg.org/dnkl/foot/issues/844).
+* Crash when setting a key- or mouse binding to the empty string
+  (https://codeberg.org/dnkl/foot/issues/851).
 
 
 ### Security

config.c

@@ -1726,6 +1726,12 @@ value_to_key_combos(struct context *ctx, int action, struct argv *argv,
 
     }
 
+    if (idx == 0) {
+        LOG_CONTEXTUAL_ERR(
+            "empty binding not allowed (set to 'none' to unmap)");
+        goto err;
+    }
+
     remove_from_key_bindings_list(bindings, action, argv);
 
     bindings->arr = xrealloc(