Prevent alloc errors from crashing in list_t

This commit changes the `list_t` api so that alloc errors can be
detected and worked around. Also fixes errors not found in 5cc7342

include/wlr/util/list.h

@@ -9,16 +9,45 @@ typedef struct {
 	void **items;
 } list_t;
 
+/**
+ * Creates a new list, may return `NULL` on failure
+ */
 list_t *list_create(void);
 void list_free(list_t *list);
 void list_foreach(list_t *list, void (*callback)(void *item));
-void list_add(list_t *list, void *item);
-void list_push(list_t *list, void *item);
-void list_insert(list_t *list, size_t index, void *item);
+/**
+ * Add `item` to the end of a list.
+ * Returns: new list length or `-1` on failure
+ */
+int list_add(list_t *list, void *item);
+/**
+ * Add `item` to the end of a list.
+ * Returns: new list length or `-1` on failure
+ */
+int list_push(list_t *list, void *item);
+/**
+ * Place `item` into index `index` in the list
+ * Returns: new list length or `-1` on failure
+ */
+int list_insert(list_t *list, size_t index, void *item);
+/**
+ * Remove an item from the list
+ */
 void list_del(list_t *list, size_t index);
+/**
+ * Remove and return an item from the end of the list
+ */
 void *list_pop(list_t *list);
+/**
+ * Get a reference to the last item of a list without removal
+ */
 void *list_peek(list_t *list);
-void list_cat(list_t *list, list_t *source);
+/**
+ * Append each item in `source` to `list`
+ * Does not modify `source`
+ * Returns: new list length or `-1` on failure
+ */
+int list_cat(list_t *list, list_t *source);
 // See qsort. Remember to use *_qsort functions as compare functions,
 // because they dereference the left and right arguments first!
 void list_qsort(list_t *list, int compare(const void *left, const void *right));