Fix Coverity warnings

Two problems were found:-
1) A useless test in scp_list.c - testing an unsigned int was >= 0.
2) Flow control issues in scp.c:scp_get_connect_session_response() meant
   that file descriptors could be leaked. A helper function has been
   used to simplify the code.

sesman/scp_list.c

@@ -273,8 +273,7 @@ scp_list_get_create_session_displays(struct set_int *alloc_displays)
         sli = (struct scp_list_item *)list_get_item(g_scp_list, i);
 
         if (SCP_LIST_ITEM_IN_USE(sli) &&
-                sli->create_session_in_progress &&
-                sli->session_display >= 0)
+                sli->create_session_in_progress)
         {
             set_int_add(alloc_displays, sli->session_display);
         }

sesman/scp_list.h

@@ -86,7 +86,8 @@ struct scp_list_item
     char start_ip_addr[MAX_PEER_ADDRSTRLEN];
     int is_admin;
     int create_session_in_progress; ///< Already handling a create_session
-    unsigned int session_display;  ///< Display allocated for create_session
+    /// Display allocated for session. This is always valid (>= 0)
+    unsigned int session_display;
 };