BUG/MEDIUM: ssl: NULL dereference in ssl_sock_load_cert_sni()

A NULL dereference can occur when inserting SNIs. In the case of
checking for duplicates, if there is already several sni_ctx with the
same key.

Fix issue #321.
diff --git a/src/ssl_sock.c b/src/ssl_sock.c
index 6a78114..90df6e7 100644
--- a/src/ssl_sock.c
+++ b/src/ssl_sock.c
@@ -2777,6 +2777,7 @@
 				LIST_DEL(&sc0->by_ckch_inst);
 				free(sc0);
 				sc0 = NULL;
+				break;
 			}
 		}