amend the CidFromReader slice extension math

The append+make slice extension idiom works, but note that append uses
the slice's length as its base. We need to append the number of bytes
required for length to reach cidLength, not the capacity.

The added test case panicked before this change, and works now:

	--- FAIL: TestReadCidsFromBuffer (0.00s)
	panic: runtime error: slice bounds out of range [:73] with capacity 64 [recovered]
		panic: runtime error: slice bounds out of range [:73] with capacity 64

	goroutine 37 [running]:
	testing.tRunner.func1.2({0x570d60, 0xc000016438})
		testing/testing.go:1203 +0x24e
	testing.tRunner.func1()
		testing/testing.go:1206 +0x218
	panic({0x570d60, 0xc000016438})
		runtime/panic.go:1038 +0x215
	github.com/ipfs/go-cid.CidFromReader({0x5b0e20, 0xc000010900})
		github.com/ipfs/go-cid/cid.go:803 +0x75f
	github.com/ipfs/go-cid.TestReadCidsFromBuffer(0xc00014ba00)
		github.com/ipfs/go-cid/cid_test.go:710 +0x625
	testing.tRunner(0xc00014ba00, 0x58af38)
		testing/testing.go:1253 +0x102
	created by testing.(*T).Run
		testing/testing.go:1300 +0x35a
	exit status 2
	FAIL	github.com/ipfs/go-cid	0.004s

cid.go

@@ -793,7 +793,7 @@ func CidFromReader(r io.Reader) (int, Cid, error) {
 	if cidLength > cap(br.dst) {
 		// If the multihash digest doesn't fit in our initial 64 bytes,
 		// efficiently extend the slice via append+make.
-		br.dst = append(br.dst, make([]byte, cidLength-cap(br.dst))...)
+		br.dst = append(br.dst, make([]byte, cidLength-len(br.dst))...)
 	} else {
 		// The multihash digest fits inside our buffer,
 		// so just extend its capacity.

cid_test.go

@@ -667,6 +667,7 @@ func TestReadCidsFromBuffer(t *testing.T) {
 		"k2cwueckqkibutvhkr4p2ln2pjcaxaakpd9db0e7j7ax1lxhhxy3ekpv",
 		"Qmf5Qzp6nGBku7CEn2UQx4mgN8TW69YUok36DrGa6NN893",
 		"zb2rhZi1JR4eNc2jBGaRYJKYM8JEB4ovenym8L1CmFsRAytkz",
+		"bafkqarjpmzuwyzltorxxezjpkvcfgqkfjfbfcvslivje2vchkzdu6rckjjcfgtkolaze6mssjqzeyn2ekrcfatkjku2vowseky3fswkfkm2deqkrju3e2",
 	}
 
 	var cids []Cid