tests/BenchmarkDecode: Skip empty pickles from tests table

In 06e06939 (encoder: Allow to specify pickle protocol version) I added
ability to add to tests error cases - object inputs that on encoding
should produce error.

For decoding we should skip those cases as there pickle.data = "", and
if not skipped it leads to

	--- FAIL: BenchmarkDecode
	    ogorek_test.go:803: unexpected # of decode steps: got 100  ; want 102

ogorek_test.go

@@ -773,6 +773,9 @@ func BenchmarkDecode(b *testing.B) {
 	npickle := 0
 	for _, test := range tests {
 		for _, pickle := range test.picklev {
+			if pickle.err != nil {
+				continue
+			}
 			// not prepending `PROTO <ver>` - decoder should be
 			// able to decode without it.
 			input = append(input, pickle.data...)