go-fuzz found panic in GenericDatumReader union parsing

[crast]

After adding some annotation code:

panic: runtime error: index out of range [recovered]
	panic: TypedValue Panic -4 [A B C D]

goroutine 1 [running]:
gopkg.in/avro%2ev0.(*GenericDatumReader).findAndSet.func1(0xc420073e90)
	/gopath/src/gopkg.in/avro.v0/datum_reader.go:475 +0x1b1
panic(0x1133ea0, 0x120ade0)
	/goroot/src/runtime/panic.go:491 +0x283
gopkg.in/avro%2ev0.(*GenericDatumReader).findAndSet(0xc42000a710, 0xc42000ac40, 0xc4200803c0, 0x11fe880, 0xc42000ac00, 0x0, 0x0)
	/gopath/src/gopkg.in/avro.v0/datum_reader.go:478 +0x4a7
gopkg.in/avro%2ev0.(*GenericDatumReader).mapRecord(0xc42000a710, 0x11fd6e0, 0xc420090070, 0x11fe880, 0xc42000ac00, 0xc42003dd90, 0x100f2c7, 0xc420073e30)
	/gopath/src/gopkg.in/avro.v0/datum_reader.go:640 +0x1dd
gopkg.in/avro%2ev0.(*GenericDatumReader).readValue(0xc42000a710, 0x11fd6e0, 0xc420090070, 0x11fe880, 0xc42000ac00, 0x199, 0x105bee9, 0xc42003de20, 0x100fa28)
	/gopath/src/gopkg.in/avro.v0/datum_reader.go:516 +0x3aa
gopkg.in/avro%2ev0.(*GenericDatumReader).Read(0xc42000a710, 0x113cc00, 0xc42000ac20, 0x11fe880, 0xc42000ac00, 0x8, 0xc42003de90)
	/gopath/src/gopkg.in/avro.v0/datum_reader.go:445 +0x23c
gopkg.in/avro%2ev0.(*anyDatumReader).Read(0xc42000a700, 0x1120fa0, 0xc42000c0c0, 0x11fe880, 0xc42000ac00, 0x1066f96, 0x5a386079)
	/gopath/src/gopkg.in/avro.v0/datum_reader.go:110 +0x12b
gopkg.in/avro.v0/fuzzes/genericreader.Fuzz(0x12d2000, 0x3, 0x200000, 0x3)
	/gopath/src/gopkg.in/avro.v0/fuzzes/genericreader/genericfuzzer.go:15 +0xef
go-fuzz-dep.Main(0x115f0d0)
	/goroot/src/go-fuzz-dep/main.go:49 +0xad
main.main()
	/gopath/src/gopkg.in/avro.v0/fuzzes/genericreader/go.fuzz.main/main.go:10 +0x2d
exit status 2

This is because we're trying to set a union with index -4