Silence valgrind uninitialized bytes complaints

Just zero out the allocated memory.

Explicitly setting struct field values isn't enough, because field alignment
means there can be several unused bytes which are written to savefile.

src/dungeon.c

@@ -1484,6 +1484,7 @@ stairway_add(coordxy x, coordxy y, boolean up, boolean isladder, d_level *dest)
 {
     stairway *tmp = (stairway *) alloc(sizeof (stairway));
 
+    (void) memset((genericptr_t) tmp, 0, sizeof (stairway));
     tmp->sx = x;
     tmp->sy = y;
     tmp->up = up;

src/mklev.c

@@ -458,6 +458,8 @@ alloc_doors(void)
     if (!gd.doors || gd.doorindex >= gd.doors_alloc) {
         int c = gd.doors_alloc + DOORINC;
         coord *doortmp = (coord *) alloc(c * sizeof(coord));
+
+        (void) memset((genericptr_t) doortmp, 0, c * sizeof(coord));
         if (gd.doors) {
             (void) memcpy(doortmp, gd.doors, gd.doors_alloc * sizeof(coord));
             free(gd.doors);