"no monster to remove" for steed knockback

Reported directly to devteam, mounted hero whose steed got hit
for knockback effect triggered impossible "no monster to remove".

In addition to fixing that, this makes a knockback attempt at a
hero who is stuck to a cursed saddle knock the hero and steed back
instead of knocking the hero out of the saddle.

mhurtle_step() should be able to use u.ux0,u.uy0 to update the
hero's old location after moving the hero in order to move the
steed, but the value was different from what was expected and the
map showed stale steed symbol when I used that.  I'm not sure what
is going on there; saving u.ux,u.uy before moving the hero worked
as intended so I didn't pursue it.

doc/fixes3-7-0.txt

@@ -1,4 +1,4 @@
-HDT-Branch: NetHack-3.7 $:$NHDT-Revision: 1.1048 $ $NHDT-Date: 1664739714 2022/10/02 19:41:54 $
+HDT-Branch: NetHack-3.7 $:$NHDT-Revision: 1.1049 $ $NHDT-Date: 1664966382 2022/10/05 10:39:42 $
 
 General Fixes and Modified Features
 -----------------------------------
@@ -1406,6 +1406,8 @@ tipping contents of one container directly into another allowed transferring
 	wands of cancellation and bags of holding or tricks that were inside
 	a sack, box, or chest into a bag of holding without blowing it up
 prevent random traps from being created inside the shops in the tourist quest
+if hero's steed got hit by knockback effect, impossible "no monster to remove"
+	would occur (plus more warnings if 'sanity_check' was On)
 
 curses: 'msg_window' option wasn't functional for curses unless the binary
 	also included tty support