fix github issue #447 - casting area-effect spell

at self when blind.  Spell targetting would let player pick
hero's own spot but casting would reject it when blind because
hero didn't sense any monster there.  The player wanted to cast
skilled fireball at self to cure being turned into slime but
wasn't allowed.  (Targetting an adjacent spot would work for
fireball, but is only feasible when telepathy reveals a monster
there.)

While testing the one-line fix, I noticed that the message line
(tty) showed stale data (autodescribe info for target spot) as
the fireball I cast (when not blind) bounced around the vicinity.
Normally that's cleared when a message is issued or the when the
next command is requested, but skilled fireball causes multiple
explosion animations before either of those situations.

src/spell.c

@@ -1,4 +1,4 @@
-/* NetHack 3.7	spell.c	$NHDT-Date: 1607980325 2020/12/14 21:12:05 $  $NHDT-Branch: NetHack-3.7 $:$NHDT-Revision: 1.110 $ */
+/* NetHack 3.7	spell.c	$NHDT-Date: 1611522041 2021/01/24 21:00:41 $  $NHDT-Branch: NetHack-3.7 $:$NHDT-Revision: 1.113 $ */
 /*      Copyright (c) M. Stephenson 1988                          */
 /* NetHack may be freely redistributed.  See license for details. */
 
@@ -1266,6 +1266,8 @@ throwspell()
     cc.y = u.uy;
     if (getpos(&cc, TRUE, "the desired position") < 0)
         return 0; /* user pressed ESC */
+    clear_nhwindow(WIN_MESSAGE); /* discard any autodescribe feedback */
+
     /* The number of moves from hero to where the spell drops.*/
     if (distmin(u.ux, u.uy, cc.x, cc.y) > 10) {
         pline_The("spell dissipates over the distance!");
@@ -1276,7 +1278,7 @@ throwspell()
         u.dx = 0;
         u.dy = 0;
         return 1;
-    } else if ((!cansee(cc.x, cc.y)
+    } else if (((cc.x != u.ux || cc.y != u.uy) && !cansee(cc.x, cc.y)
                 && (!(mtmp = m_at(cc.x, cc.y)) || !canspotmon(mtmp)))
                || IS_STWALL(levl[cc.x][cc.y].typ)) {
         Your("mind fails to lock onto that location!");