bpf: Handle NULL in bpf_local_storage_free.

During OOM bpf_local_storage_alloc() may fail to allocate 'storage' and
call to bpf_local_storage_free() with NULL pointer will cause a crash like:
[ 271718.917646] BUG: kernel NULL pointer dereference, address: 00000000000000a0
[ 271719.019620] RIP: 0010:call_rcu+0x2d/0x240
[ 271719.216274] bpf_local_storage_alloc+0x19e/0x1e0
[ 271719.250121] bpf_local_storage_update+0x33b/0x740

Fixes: 7e30a8477b0b ("bpf: Add bpf_local_storage_free()")
Signed-off-by: Alexei Starovoitov <[e-mail]>
Signed-off-by: Andrii Nakryiko <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

kernel/bpf/bpf_local_storage.c

···
 				   struct bpf_local_storage_map *smap,
 				   bool bpf_ma, bool reuse_now)
 {
+	if (!local_storage)
+		return;
+
 	if (!bpf_ma) {
 		__bpf_local_storage_free(local_storage, reuse_now);
 		return;