did you tried 

server_name _; 

already?

did you chained the certs for a.com  / c.com in the correct order?

see http://nginx.org/en/docs/http/configuring_https_servers.html / 
An SSL certificate with several names

Posted at Nginx Forum: 
http://forum.nginx.org/read.php?2,253929,253946#msg-253946

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