BUG/MINOR: ssl: always check for ssl connection before getting its XPRT context

BUG/MINOR: ssl: always check for ssl connection before getting its XPRT context

In several SSL functions, the XPRT context is retrieved before any check on the
connection. In the function ssl_sock_is_ssl(), a test suggests the connection
may be null. So, it is safer to test the ssl connection before retrieving its
XPRT context. It removes any ambiguities and prevents possible null pointer
dereferences.

This patch fixes the issue #265. It must be backported to 2.0.