Conversion from Data to ASN1 crashes

[brabanod]

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)

[mengoreo]

Same here with ECPublicKey