Fix: panic when childer is nil

[ajnavarro]

Context: ipfs/kubo#9063

When stressing go-mfs over fuzz testing, we can cause nil pointer exceptions like:

testing.go:1349: panic: runtime error: invalid memory address or nil pointer dereference
            goroutine 6908 [running]:
            runtime/debug.Stack()
                /usr/local/go/src/runtime/debug/stack.go:24 +0x90
            testing.tRunner.func1()
                /usr/local/go/src/testing/testing.go:1349 +0x1f2
            panic({0xa6ec60, 0xefe2e0})
                /usr/local/go/src/runtime/panic.go:838 +0x207
            github.com/ipfs/go-unixfs/hamt.(*childer).has(...)
                /home/ajnavarro/go/pkg/mod/github.com/ipfs/go-unixfs@v0.4.0/hamt/hamt.go:886
            github.com/ipfs/go-unixfs/hamt.(*Shard).swapValue(0xc0029067e0, {0xbc1440, 0xc000024088}, 0xc0000e5348, {0xc00014e001, 0xf10}, 0xc002462420)
                /home/ajnavarro/go/pkg/mod/github.com/ipfs/go-unixfs@v0.4.0/hamt/hamt.go:623 +0xa3
            github.com/ipfs/go-unixfs/hamt.(*Shard).swapValue(0xc002906770, {0xbc1440, 0xc000024088}, 0xc0000e5348, {0xc00014e001, 0xf10}, 0xc002462420)
                /home/ajnavarro/go/pkg/mod/github.com/ipfs/go-unixfs@v0.4.0/hamt/hamt.go:689 +0x745
            github.com/ipfs/go-unixfs/hamt.(*Shard).SetLink(0xc002906770, {0xbc1440, 0xc000024088}, {0xc00014e001, 0xf10}, 0xc001fbaf90)
                /home/ajnavarro/go/pkg/mod/github.com/ipfs/go-unixfs@v0.4.0/hamt/hamt.go:248 +0x1f7
            github.com/ipfs/go-unixfs/io.(*BasicDirectory).switchToSharding(0xc00007ca60, {0xbc1440, 0xc000024088})
                /home/ajnavarro/go/pkg/mod/github.com/ipfs/go-unixfs@v0.4.0/io/directory.go:337 +0x2db
            github.com/ipfs/go-unixfs/io.(*DynamicDirectory).AddChild(0xc0002efdd0, {0xbc1440, 0xc000024088}, {0xc0044f9629, 0x2}, {0xbc32b8, 0xc0027f13e0})
                /home/ajnavarro/go/pkg/mod/github.com/ipfs/go-unixfs@v0.4.0/io/directory.go:569 +0x113
            github.com/ipfs/go-mfs.(*Directory).Mkdir(0xc0001a4800, {0xc0044f9629, 0x2})
                /home/ajnavarro/workspace/go-mfs/dir.go:316 +0x516
            github.com/ipfs/go-mfs.Mkdir(0xc0002efdb0, {0xc0044f9629, 0x2}, {0x0?, 0x0?, {0x0?, 0x0?}})
                /home/ajnavarro/workspace/go-mfs/ops.go:175 +0x31f
            github.com/ipfs/go-mfs.FuzzMkdirAndWriteConcurrently.func1(0xc000102680, 0xd0?, 0x57?, {0xc0044f9629, 0x2}, {0x0, 0x0}, {0xc0044f9650, 0x1, 0x8})
                /home/ajnavarro/workspace/go-mfs/mfs_test.go:1540 +0x8d
            reflect.Value.call({0xa75ae0?, 0xc0002efde0?, 0x13?}, {0xaf8ffc, 0x4}, {0xc002548a80, 0x6, 0x8?})
                /usr/local/go/src/reflect/value.go:556 +0x845
            reflect.Value.Call({0xa75ae0?, 0xc0002efde0?, 0x514?}, {0xc002548a80, 0x6, 0x8})
                /usr/local/go/src/reflect/value.go:339 +0xbf
            testing.(*F).Fuzz.func1.1(0x0?)
                /usr/local/go/src/testing/fuzz.go:337 +0x231
            testing.tRunner(0xc000102680, 0xc0000d9290)
                /usr/local/go/src/testing/testing.go:1439 +0x102
            created by testing.(*F).Fuzz.func1
                /usr/local/go/src/testing/fuzz.go:324 +0x5b8

This PR patches that problem.

I need some experienced eyes to check that what I did is 100% correct.