Update cours_21.md (modif fonction qt_create)

slides/cours_21.md

@@ -197,11 +197,12 @@ arbre creer_arbre(prof)
 
 ```C
 node *qt_create(int depth) {
+    if (depth <= 0) {
+        return NULL;
+    }
     node *n = calloc(1, sizeof(node));
-    if (depth > 0) {
-        for (int i = 0; i < 4; ++i) {
-            n->child[i] = qt_create(depth-1);
-        }
+    for (int i = 0; i < 4; ++i) {
+        n->child[i] = qt_create(depth-1);
     }
     return n;
 }