debug(tableIndex): add debug info for initIndex

[NamanJain8]

Fixes DGRAPH-2841

This PR adds debug information for the following crash. We suspect that it may be due to unsynced mmap (#1625), or because of mishandling of sst files.

panic: runtime error: index out of range [3] with length 0
panic: runtime error: index out of range [3] with length 0

goroutine 187 [running]:
github.com/google/flatbuffers/go.GetInt32(...)
	/go/pkg/mod/github.com/google/flatbuffers@v1.12.0/go/encode.go:85
github.com/google/flatbuffers/go.GetUOffsetT(...)
	/go/pkg/mod/github.com/google/flatbuffers@v1.12.0/go/encode.go:121
github.com/dgraph-io/badger/v2/fb.GetRootAsTableIndex(...)
	/go/pkg/mod/github.com/dgraph-io/badger/v2@v2.0.1-rc1.0.20201214114056-bcfae6104545/fb/TableIndex.go:14
github.com/dgraph-io/badger/v2/table.(*Table).readTableIndex(0xc0005dc000, 0x0, 0x8, 0xc0001fe210)
	/go/pkg/mod/github.com/dgraph-io/badger/v2@v2.0.1-rc1.0.20201214114056-bcfae6104545/table/table.go:654 +0x234
github.com/dgraph-io/badger/v2/table.(*Table).initIndex(0xc0005dc000, 0xc000075ac0, 0x7f8be018d4c0, 0xc00004a000)
	/go/pkg/mod/github.com/dgraph-io/badger/v2@v2.0.1-rc1.0.20201214114056-bcfae6104545/table/table.go:412 +0x285
github.com/dgraph-io/badger/v2/table.(*Table).initBiggestAndSmallest(0xc0005dc000, 0x0, 0x0)
	/go/pkg/mod/github.com/dgraph-io/badger/v2@v2.0.1-rc1.0.20201214114056-bcfae6104545/table/table.go:351 +0x5e
github.com/dgraph-io/badger/v2/table.OpenTable(0xc000182280, 0x0, 0x200000, 0x0, 0x0, 0x3f847ae147ae147b, 0x1000, 0x0, 0x1, 0xc000560080, ...)
	/go/pkg/mod/github.com/dgraph-io/badger/v2@v2.0.1-rc1.0.20201214114056-bcfae6104545/table/table.go:312 +0x22d
github.com/dgraph-io/badger/v2.newLevelsController.func1(0xc00048c7c0, 0xc0001f41e0, 0xc000098c00, 0xc0001f41b8, 0xc00020e2c0, 0x7, 0x7, 0xc0001f4220, 0xc, 0x1d6b606, ...)
	/go/pkg/mod/github.com/dgraph-io/badger/v2@v2.0.1-rc1.0.20201214114056-bcfae6104545/levels.go:150 +0x205
created by github.com/dgraph-io/badger/v2.newLevelsController
	/go/pkg/mod/github.com/dgraph-io/badger/v2@v2.0.1-rc1.0.20201214114056-bcfae6104545/levels.go:129 +0x645

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This change is