directio: Check if buffers are set. (#13440)
Check if directio buffers have actually been fetched and prevent errors on double Close. Return error on Read after Close.
Fixes
```
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0xf8582f]
goroutine 210 [running]:
github.com/minio/minio/internal/ioutil.(*ODirectReader).Read(0xc0054f8320, {0xc0014560b0, 0xa8, 0x44d012})
github.com/minio/minio/internal/ioutil/odirect_reader.go:88 +0x10f
io.ReadAtLeast({0x428c5c0, 0xc0054f8320}, {0xc0014560b0, 0xa8, 0xa8}, 0xa8)
io/io.go:328 +0x9a
io.ReadFull(...)
io/io.go:347
github.com/minio/minio/internal/ioutil.ReadFile({0xc001bf60e0, 0x6})
github.com/minio/minio/internal/ioutil/read_file.go:48 +0x19b
github.com/minio/minio/cmd.(*FSObjects).scanBucket.func1({{0xc00444e1e0, 0x4d}, 0x0, {0xc0040cf240, 0xe}, {0xc0040cf24f, 0x18}, {0xc0040cf268, 0x18}, 0x0, ...})
github.com/minio/minio/cmd/fs-v1.go:366 +0x1ea
github.com/minio/minio/cmd.(*folderScanner).scanFolder.func1({0xc00474a6a8, 0xc0065d6793}, 0x0)
github.com/minio/minio/cmd/data-scanner.go:494 +0xb15
github.com/minio/minio/cmd.readDirFn({0xc002803e80, 0x34}, 0xc000670270)
github.com/minio/minio/cmd/os-readdir_unix.go:172 +0x638
github.com/minio/minio/cmd.(*folderScanner).scanFolder(0xc002deeb40, {0x42dc9d0, 0xc00068cbc0}, {{0xc001c6e2d0, 0x27}, 0xc0023db8e0, 0x1}, 0xc0001c7ab0)
github.com/minio/minio/cmd/data-scanner.go:427 +0xa8f
github.com/minio/minio/cmd.(*folderScanner).scanFolder.func2({{0xc001c6e2d0, 0x27}, 0xc0023db8e0, 0x27})
github.com/minio/minio/cmd/data-scanner.go:549 +0xd0
github.com/minio/minio/cmd.(*folderScanner).scanFolder(0xc002deeb40, {0x42dc9d0, 0xc00068cbc0}, {{0xc0013fa9e0, 0xe}, 0x0, 0x1}, 0xc000670dd8)
github.com/minio/minio/cmd/data-scanner.go:623 +0x205d
github.com/minio/minio/cmd.scanDataFolder({_, _}, {_, _}, {{{0xc0013fa9e0, 0xe}, 0x802, {0x210f15d2, 0xed8f903b8, 0x5bc0e80}, ...}, ...}, ...)
github.com/minio/minio/cmd/data-scanner.go:333 +0xc51
github.com/minio/minio/cmd.(*FSObjects).scanBucket(_, {_, _}, {_, _}, {{{0xc0013fa9e0, 0xe}, 0x802, {0x210f15d2, 0xed8f903b8, ...}, ...}, ...})
github.com/minio/minio/cmd/fs-v1.go:364 +0x305
github.com/minio/minio/cmd.(*FSObjects).NSScanner(0x42dc9d0, {0x42dc9d0, 0xc00068cbc0}, 0x0, 0xc003bcfda0, 0x802)
github.com/minio/minio/cmd/fs-v1.go:307 +0xa16
github.com/minio/minio/cmd.runDataScanner({0x42dc9d0, 0xc00068cbc0}, {0x436a6c0, 0xc000bfcf50})
github.com/minio/minio/cmd/data-scanner.go:150 +0x749
created by github.com/minio/minio/cmd.initDataScanner
github.com/minio/minio/cmd/data-scanner.go:73 +0xb0
```
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