of: overlay: fix null pointer dereferencing in find_dup_cset_node_entry() and find_dup_cset_prop()

When kmalloc() fail to allocate memory in kasprintf(), fn_1 or fn_2 will
be NULL, and strcmp() will cause null pointer dereference.

Fixes: 2fe0e876

 ("of: overlay: check prevents multiple fragments touching same property")
Signed-off-by: ruanjinjie <ruanjinjie@huawei.com>
Link: https://lore.kernel.org/r/20221211023337.592266-1-ruanjinjie@huawei.com
Signed-off-by: Rob Herring <robh@kernel.org>