go-fuzz found crash in SpecificReader mapArray

[crast]

panic: reflect.MakeSlice: negative len

goroutine 1 [running]:
reflect.MakeSlice(0x11fc3c0, 0x1124bc0, 0x8000000000000000, 0x8000000000000000, 0x1124bc0, 0xc42000a360, 0x97)
	/goroot/src/reflect/value.go:2052 +0x2d7
gopkg.in/avro%2ev0.sDatumReader.mapArray(0x11f92c0, 0xc42000a240, 0x1124bc0, 0xc420090100, 0x197, 0x11fa880, 0xc42000a340, 0xc42003fc28, 0x11010c7, 0x122ba30, ...)
	/gopath/src/gopkg.in/avro.v0/datum_reader.go:245 +0x7fe
gopkg.in/avro%2ev0.sDatumReader.readValue(0x11f92c0, 0xc42000a240, 0x1124bc0, 0xc420090100, 0x197, 0x11fa880, 0xc42000a340, 0x197, 0x0, 0x0, ...)
	/gopath/src/gopkg.in/avro.v0/datum_reader.go:198 +0x26e
gopkg.in/avro%2ev0.sDatumReader.findAndSet(0x111fae0, 0xc420090100, 0x16, 0xc420080320, 0x11fa880, 0xc42000a340, 0x105c8d5, 0x3)
	/gopath/src/gopkg.in/avro.v0/datum_reader.go:169 +0x12a
gopkg.in/avro%2ev0.sDatumReader.fillRecord(0x11f96e0, 0xc42008e070, 0x111fae0, 0xc420090100, 0x16, 0x11fa880, 0xc42000a340, 0xc42003fe90, 0x100fa28)
	/gopath/src/gopkg.in/avro.v0/datum_reader.go:403 +0xec
gopkg.in/avro%2ev0.(*SpecificDatumReader).Read(0xc42000e7a0, 0x111fae0, 0xc420090100, 0x11fa880, 0xc42000a340, 0x1066f96, 0x5a38475a)
	/gopath/src/gopkg.in/avro.v0/datum_reader.go:154 +0x290
gopkg.in/avro.v0/fuzzes/specific-reader-complex.Fuzz(0x12ce000, 0xb, 0x200000, 0xc42003ff70)
	/gopath/src/gopkg.in/avro.v0/fuzzes/specific-reader-complex/fuzzer.go:17 +0xef
go-fuzz-dep.Main(0x115b5d8)
	/goroot/src/go-fuzz-dep/main.go:49 +0xad
main.main()
	/gopath/src/gopkg.in/avro.v0/fuzzes/specific-reader-complex/go.fuzz.main/main.go:10 +0x2d
exit status 2

From Input:

	"\xff\xff\xff\xff\xff\xff\xff\xff\xff\x010"