runtime error: slice bounds out of range

Hello,

Sometimes unexpectedly get the following error

```
panic: runtime error: slice bounds out of range [:1378] with capacity 1315

github.com/google/certificate-transparency-go/asn1.parseField({0x9a98e0?, 0xc004b1b800?, 0xb?}, {0xc0119ec100, 0x562, 0x523}, 0x0, {0x0, 0x0, 0x0, ...})
#011/root/go/pkg/mod/github.com/google/certificate-transparency-go@v1.1.3/asn1/asn1.go:940 +0x31de
github.com/google/certificate-transparency-go/asn1.UnmarshalWithParams({0xc0119ec100, 0x562, 0x523}, {0x937a20?, 0xc004b1b800?}, {0x0, 0x0})
#011/root/go/pkg/mod/github.com/google/certificate-transparency-go@v1.1.3/asn1/asn1.go:1190 +0x1b7
github.com/google/certificate-transparency-go/asn1.Unmarshal(...)
#011/root/go/pkg/mod/github.com/google/certificate-transparency-go@v1.1.3/asn1/asn1.go:1183
github.com/google/certificate-transparency-go/x509.ParseCertificate({0xc0119ec100, 0x562, 0x523})
#011/root/go/pkg/mod/github.com/google/certificate-transparency-go@v1.1.3/x509/x509.go:2116 +0x95
```

Start at asn1/asn1.go:940, where the slice-bounds panic occurs, and trace the call from x509/x509.go:2116 through ParseCertificate. Use the reported certificate input or obtain a reproducible case, then verify that malformed or unexpected data no longer causes this panic and that certificate parsing still behaves correctly.

Written by the indexing model from the issue text.