Kernel: Handle OOM of file system in sys$mount

Author: https://github.com/bgianfo
Commit: 7c0b2eb0f5
Pull-request: https://github.com/SerenityOS/serenity/pull/7624

Kernel/Syscalls/mount.cpp

@@ -95,6 +95,9 @@ KResultOr<int> Process::sys$mount(Userspace<const Syscall::SC_mount_params*> use
         return ENODEV;
     }
 
+    if (!fs)
+        return ENOMEM;
+
     if (!fs->initialize()) {
         dbgln("mount: failed to initialize {} filesystem, fd={}", fs_type, source_fd);
         return ENODEV;