IsoApplet: ECDSA signatures - only strip excess zeroes

It is required to strip excessive zeroes returned by some Java Cards when removeing the ASN1 structural information for PKCS#11 so that the x/y can be calculated by dividing the signature length by 2. However, the leading zero may only be stripped if it is excessive (outside the field length). Otherwise generated signatures are wrong in rare cases (1 out of 256).