Conversion from Data to ASN1 crashes

**_Description_**:
When initializing `ECSignature` with wrong data, the conversion in `ASN1.toASN1Element` crashes for at
```
default: // octet string
let (length, lengthOfLength) = readLength(data: data.advanced(by: 1))
```

**_Reproduce_**:
```
let signed = try ECSignature(asn1: Data(base64Encoded: "abcd")!)
```

**_Expected:_**
`ECSignature` throws an error.

**_Actual:_**
Program crashes with the following message:
`Thread 1: EXC_BAD_INSTRUCTION (code=EXC_I386_INVOP, subcode=0x0)`