tests: Fix BenchmarkEncode

After 6e5e403e (encoder: Fix GLOBAL emission wrt module/name with \n)
BenchmarkEncode started to fail with

    --- FAIL: BenchmarkEncode
        ogorek_test.go:1003: protocol 0-3: global: module & name must be string without \n

-> Fix it by taking as input only those testdata objects that are marked
to encode without errors.

ogorek_test.go

@@ -1000,9 +1000,11 @@ func BenchmarkEncode(b *testing.B) {
 	input := make([]interface{}, 0)
 	approxOutSize := 0
 	for _, test := range tests {
+		if test.picklev[0].err == nil {
 			input = append(input, test.objectIn)
 			approxOutSize += len(test.picklev[0].data)
 		}
+	}
 
 	buf := bytes.NewBuffer(make([]byte, approxOutSize))