bpf: Fix NULL dereference in bpf_task_storage

In bpf_pid_task_storage_update_elem(), it missed to
test the !task_storage_ptr(task) which then could trigger a NULL
pointer exception in bpf_local_storage_update().

Fixes: 4cf1bc1f

 ("bpf: Implement task local storage")
Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Tested-by: Roman Gushchin <guro@fb.com>
Acked-by: KP Singh <kpsingh@google.com>
Link: https://lore.kernel.org/bpf/20201112001919.2028357-1-kafai@fb.com