Fix q_size to return correct queue length

Previously, q_size always returned -1, which was incorrect.
Now, it correctly iterates through the list and returns the actual
length. Additionally, a null check for head was added to return 0.

Change-Id: Ic9af6472a5516593a68a3a8e0d3f0746b8dc43f7

Author: yy214123

queue.c

@@ -46,7 +46,15 @@ element_t *q_remove_tail(struct list_head *head, char *sp, size_t bufsize)
 /* Return number of elements in queue */
 int q_size(struct list_head *head)
 {
-    return -1;
+    if (!head)
+        return 0;
+
+    int len = 0;
+    struct list_head *li;
+
+    list_for_each (li, head)
+        len++;
+    return len;
 }
 
 /* Delete the middle node in queue */