ngx_voms: segmentation fault if plain http request

The storm2 testsuite shows an error in the error.log of nginx:

storm_1    | 2020/01/13 05:41:40 [alert] 6#6: worker process 7 exited on signal 11

Looking at the debug log it seems that there is a plain http request that leads to code inside the voms module which is not prepared for it. Indeed there is a missing check when dereferencing ssl in r->connection->ssl->connection.

For the debug log see https://baltig.infn.it/storm2/storm2/issues/79#note_26695