sd-bus: fix use of ERRNO_IS_DISCONNECT()

Given that ERRNO_IS_DISCONNECT() also matches positive values,
make sure this macro is not called with arguments that do not have
errno semantics.

In this case the argument passed to ERRNO_IS_DISCONNECT() is the value
returned by bus_socket_process_watch_bind(), bus_socket_process_opening(),
and bus_socket_process_authenticating() which can legitimately return
positive values without errno semantics, so fix this by moving the
ERRNO_IS_DISCONNECT() invocation to the branch where the return value
is known to be negative.

(cherry picked from commit bb228f0)

Author: ldv-alt

src/libsystemd/sd-bus/sd-bus.c

@@ -3249,11 +3249,13 @@ static int bus_process_internal(sd_bus *bus, sd_bus_message **ret) {
                 assert_not_reached();
         }
 
-        if (ERRNO_IS_DISCONNECT(r)) {
-                bus_enter_closing(bus);
-                r = 1;
-        } else if (r < 0)
-                return r;
+        if (r < 0) {
+                if (ERRNO_IS_DISCONNECT(r)) {
+                        bus_enter_closing(bus);
+                        r = 1;
+                } else
+                        return r;
+        }
 
         if (ret)
                 *ret = NULL;