Unbreak ASN.1 indefinite length encoding.

Unbreak ASN.1 indefinite length encoding.

In r1.25 of tasn_enc.c a check was added to ensure that asn1_ex_i2c()
returned the same value on both calls, however in the ndef case the len
variable gets changed between calls. Keep a copy of the original value to
test against.

Issue reported by niklas, who encountered a test failure in rust-openssl.

ok miod@ tb@