Fix q_remove function for memory leak

For the previous version of the function, when the input char pointer is
NULL the function will return NULL. Change the return of the function to
the correct pointer.

Change-Id: I09fed262bb2e364a92dc9c30a6f4a917e2c0ad68

Author: charliechiou

queue.c

@@ -98,7 +98,7 @@ element_t *q_remove_head(struct list_head *head, char *sp, size_t bufsize)
     list_del(&first->list);
 
     if (!sp) {
-        return NULL;
+        return first;
     }
 
     size_t len = strlen(first->value);
@@ -121,7 +121,7 @@ element_t *q_remove_tail(struct list_head *head, char *sp, size_t bufsize)
     list_del(&tail->list);
 
     if (!sp) {
-        return NULL;
+        return tail;
     }
 
     size_t len = strlen(tail->value);