Fix missing empty list checks

circ_list.c

@@ -27,6 +27,10 @@ unsigned int list_count(element *head) {
 }
 
 element *list_move(element *head) {
+    if (list_empty(head)) {
+        return NULL;
+    }
+
     return head->next;
 }
 
@@ -59,6 +63,10 @@ element *list_insert_before(element *head, int data) {
 }
 
 element *list_search(element *head, int data, int (*compare)(int, int)) {
+    if (list_empty(head)) {
+        return NULL;
+    }
+
     element *iterator = head;
 
     do {
@@ -91,6 +99,10 @@ element *list_remove(element *head, int data, int (*compare)(int, int)) {
 }
 
 void list_free(element *head) {
+    if (list_empty(head)) {
+        return;
+    }
+
     element *iterator = head;
 
     do {
@@ -101,6 +113,10 @@ void list_free(element *head) {
 }
 
 void list_process(element *head, int (*action)(int)) {
+    if (list_empty(head)) {
+        return;
+    }
+
     element *iterator = head;
 
     do {