_findCert bugfix for multiple signers with different certificats

[coachaac]

this pull request is the next step to my bug report. Thanks to you i patch my certificate mismatch and then i found thoses bugs when trying to validate my signedData.

my case is this one :

• data is signed by 2 signers with their own certificates
• after signing, i use KJUR.asn1.cms.CMSUtil.verifySignedData to make sure every thing is ok.
• but it crash in _findCert.

There are 2 bugs :

• aIdx is not set when result.certs is already defined.
• the second for loop is "looping" on x instead of aCert[].