Allocate rects dynamically

... instead of hard-coding them to 50. New allocated value is
(COLNO*ROWNO)/30, which is slightly higher (56), and that formula
seems to work for hypothetical larger maps too.

src/save.c

@@ -1082,6 +1082,7 @@ freedynamicdata(void)
     free_waterlevel();
     free_dungeons();
     free_CapMons();
+    free_rect();
 
     /* some pointers in iflags */
     if (iflags.wc_font_map)