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: 7e30a847

 ("bpf: Add bpf_local_storage_free()")
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20230412171252.15635-1-alexei.starovoitov@gmail.com