Address panics in parseBinaryFloat, add fuzzer

[TomSellers]

This PR addresses two indexing related panics in parseBinaryFloat. It also adds a fuzzer for DecodePacketErr to help identify similar issues in the future.

The fuzzer can be run using the following command:
go test -v -fuzz ^FuzzDecodePacket$ .

The following reproducer will trigger the panics. Uncomment the second data= block to trigger that one.

package main

import (
	ber "github.com/go-asn1-ber/asn1-ber"
)

func main() {
	// panic: runtime error: slice bounds out of range [:2] with capacity 1
	// parseBinaryFloat - [email protected]/real.go:98
	// https://github.com/go-asn1-ber/asn1-ber/blob/master/real.go#L98-L99
	// Root cause: failure to check length of 'v' before before indexing into it
	data := []byte{0x09, 0x02, 0x85, 0x30}

	// panic: runtime error: index out of range [0] with length 0
	// parseBinaryFloat - [email protected]/real.go:92
	// https://github.com/go-asn1-ber/asn1-ber/blob/3c992f0d6bf4482d85368da48d2a37fa6a5a120b/real.go#L92
	// Root cause: failure to check length of slice 'v' before indexing into it
	//data = []byte{0x09, 0x01, 0xcf}

	_ = ber.DecodePacket(data)
}

[TomSellers]

I've updated the logic to add build tags so that the fuzz tests won't generate errors when being processed by older versions of Go.

[johnweldon]

Thank you @TomSellers