BUG/MINOR: ssl: segv on startup when AKID but no keyid

BUG/MINOR: ssl: segv on startup when AKID but no keyid

In bug #959 it was reported that haproxy segfault on startup when trying
to load a certifcate which use the X509v3 AKID extension but without the
keyid field.

This field is not mandatory and could be replaced by the serial or the
DirName.

For example:

   X509v3 extensions:
       X509v3 Basic Constraints:
           CA:FALSE
       X509v3 Subject Key Identifier:
           42:7D:5F:6C:3E:0D:B7:2C:FD:6A:8A:32:C6:C6:B9:90:05:D1:B2:9B
       X509v3 Authority Key Identifier:
           DirName:/O=HAProxy Technologies/CN=HAProxy Test Intermediate CA
           serial:F2:AB:C1:41:9F:AB:45:8E:86:23:AD:C5:54:ED:DF:FA

This bug was introduced by 70df7b ("MINOR: ssl: add "issuers-chain-path" directive").

This patch must be backported as far as 2.2.