LibC: Fix `scandir` not checking for allocation failure

Userland/Libraries/LibC/dirent.cpp

@@ -278,6 +278,9 @@ int scandir(const char* dir_name,
 
     const int size = tmp_names.size();
     auto** names = static_cast<struct dirent**>(kmalloc_array(size, sizeof(struct dirent*)));
+    if (names == nullptr) {
+        return -1;
+    }
     for (auto i = 0; i < size; i++) {
         names[i] = tmp_names[i];
     }