Add assertion for Coverity.

Coverity thinks this value might be NULL but that should not be possible because of the TRY...CATCH block.

src/common/io/tls/client.c

@@ -304,6 +304,7 @@ tlsClientOpen(THIS_VOID)
         while (retry);
 
         // Authenticate TLS session
+        ASSERT(result != NULL);
         ioSessionAuthenticatedSet(result, tlsClientAuth(this, tlsSession));
 
         // Move session