coccinelle: add reallocarray() coccinelle script

Let's systematically make use of reallocarray() whereever we invoke
realloc() with a product of two values.

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

@@ -1400,7 +1400,7 @@ static int message_push_fd(sd_bus_message *m, int fd) {
         if (copy < 0)
                 return -errno;
 
-        f = realloc(m->fds, sizeof(int) * (m->n_fds + 1));
+        f = reallocarray(m->fds, sizeof(int), m->n_fds + 1);
         if (!f) {
                 m->poisoned = true;
                 safe_close(copy);

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

@@ -1223,7 +1223,7 @@ int bus_socket_read_message(sd_bus *bus) {
                                         return -EIO;
                                 }
 
-                                f = realloc(bus->fds, sizeof(int) * (bus->n_fds + n));
+                                f = reallocarray(bus->fds, bus->n_fds + n, sizeof(int));
                                 if (!f) {
                                         close_many((int*) CMSG_DATA(cmsg), n);
                                         return -ENOMEM;