sd-bus: store the strv size when extending it

So strv_push_with_size() doesn't have to recalculate the size every
time.

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

@@ -4331,6 +4331,7 @@ int bus_message_get_blob(sd_bus_message *m, void **buffer, size_t *sz) {
 _public_ int sd_bus_message_read_strv_extend(sd_bus_message *m, char ***l) {
         char type;
         const char *contents, *s;
+        size_t n;
         int r;
 
         assert(m);
@@ -4347,9 +4348,10 @@ _public_ int sd_bus_message_read_strv_extend(sd_bus_message *m, char ***l) {
         if (r <= 0)
                 return r;
 
+        n = strv_length(*l);
         /* sd_bus_message_read_basic() does content validation for us. */
         while ((r = sd_bus_message_read_basic(m, *contents, &s)) > 0) {
-                r = strv_extend(l, s);
+                r = strv_extend_with_size(l, &n, s);
                 if (r < 0)
                         return r;
         }