Fix crash during restore when mounted

The new "riding blocks stealth" code was reading u.usteed at an
early point in the restore process, before its value was reliable
(dorecover()->restgamestate()->set_uasmon()->steed_vs_stealth());
because restgamestate() happens prior to restlevelstate(), the
value of u.usteed will be a stale pointer from some previous game,
and attempting to determine whether the steed is flying will crash
the game.

steed_vs_stealth() doesn't actually need to be called during the
restore process (because BStealth is saved in the save file), so
this can be easily fixed by omitting the call to it during the
restore.

src/polyself.c

@@ -105,7 +105,12 @@ set_uasmon(void)
     PROPSET(BLINDED, !haseyes(mdat));
 #undef PROPSET
 
-    float_vs_flight(); /* maybe toggle (BFlying & I_SPECIAL) */
+    /* whether the player is flying/floating depends on their steed,
+       which won't be known during the restore process: but BFlying
+       and BStealth should be set correctly already in that case, so
+       there's nothing to do */
+    if (!gp.program_state.restoring)
+        float_vs_flight(); /* maybe toggle (BFlying & I_SPECIAL) */
     polysense();
 
 #ifdef STATUS_HILITES