[GRASS5] r.patch non-functional

[Markus Neteler]

After receiving a user-complaint about r.patch I
have tried it myself right now (HEAD).

In fact r.patch is no longer functional, the
resulting map only contains the first image.

It seems to be related to a library change as the
r.patch code is almost untouched.
This should be fixed for 5.0.2.

Markus