Fix coding of LE in Case 2E in xchgAPDUExtended
Fix coding of LE in Case 2E in xchgAPDUExtended

Commit 1497409136ca77648c157723d2efc512a88cc395 fixed the coding of LC if LC==0, but wrongly encodes the LE field: In the case 2E (LC == 0, LE > 0), the LE value has to be coded on 3 bytes with a leading zero byte. This commit resolves this issue.
google/u2f-ref-code,google/u2f-ref-code,google/u2f-ref-code,google/u2f-ref-code,google/u2f-ref-code