stailq: add non empty list assertion to stailq_shift

stailq_shift expects list to be non empty. Let's add assertion.

This patch addresses coverity complains 1526815 and 1526816 (dup).
https://scan7.scan.coverity.com/reports.htm#v43693/p13437/fileInstanceId=129122176&defectInstanceId=18601010&mergedDefectId=1526815

Closes #7913

NO_DOC=code health
NO_CHANGELOG=code health
NO_TEST=code health

src/lib/salad/stailq.h

@@ -79,6 +79,15 @@ stailq_create(struct stailq *head)
 	head->last = &head->first;
 }
 
+/**
+ * return TRUE if list is empty
+ */
+static inline int
+stailq_empty(struct stailq *head)
+{
+	return head->first.value == NULL;
+}
+
 /**
  * Add an item to list head
  */
@@ -97,6 +106,7 @@ stailq_add(struct stailq *head, struct stailq_entry *item)
 inline static struct stailq_entry *
 stailq_shift(struct stailq *head)
 {
+	assert(!stailq_empty(head));
 	struct stailq_entry_ptr shift = head->first;
 	head->first = head->first.value->next;
 	if (head->first.value == NULL)
@@ -156,15 +166,6 @@ stailq_next(struct stailq_entry *item)
 	return item->next.value;
 }
 
-/**
- * return TRUE if list is empty
- */
-inline static int
-stailq_empty(struct stailq *head)
-{
-	return head->first.value == NULL;
-}
-
 /*
  * Singly-linked Tail queue functions.
  */