dirmngr: Fix verification of ECDSA signed CRLs.

* dirmngr/crlcache.c (finish_sig_check): Use raw value for the data.

This had the usual signed/unsigned problem. By using the modern form
we enforce Libgcrypt internal parsing as unsigned integer.

(cherry picked from commit 868dabb4027a03f4ce39be3c143b480bccde1a63)