Fixes for degenerate RNG

Assume a degenerate RNG that always returns the same number.
With these fixes, the game starts and is almost playable.
There are still some places that will go into infinite loop, eg.
the mines level generation.

src/mkmaze.c

@@ -1035,6 +1035,7 @@ const char *s;
         mazexy(&mm);
         mkstairs(mm.x, mm.y, 0, (struct mkroom *) 0); /* down */
     } else { /* choose "vibrating square" location */
+        int trycnt = 0;
 #define x_maze_min 2
 #define y_maze_min 2
 /*
@@ -1064,6 +1065,8 @@ const char *s;
             y = rn1(y_range, y_maze_min + INVPOS_Y_MARGIN + 1);
             /* we don't want it to be too near the stairs, nor
                to be on a spot that's already in use (wall|trap) */
+            if (++trycnt > 1000)
+                break;
         } while (x == xupstair || y == yupstair /*(direct line)*/
                  || abs(x - xupstair) == abs(y - yupstair)
                  || distmin(x, y, xupstair, yupstair) <= INVPOS_DISTANCE