Make pline_mon work if youmonst is passed to it

I was working on another patch involving a message that could be printed
for either a monster or the player (using a struct monst * variable that
either holds the monster or &gy.youmonst), but wasn't able to easily use
pline_mon for the message since the mx and my of youmonst aren't kept
updated as the hero moves. (In my testing, they were always 0, but it's
not clear if they will remain 0 throughout the game, or if that's a bad
assumption to make.)

Allow this in the future by checking for youmonst in pline_mon and
setting the coordinates to 0,0 explicitly so no relative coordinate
message gets printed when it's about the hero.

I'm not sure if it's a reasonable assumption that no messages that could
ever be passed to pline_mon for a player would ever need to note
"(here)" when accessiblemsg is turned on. If that's the case, the
correct thing would be to set the coordinates to u.ux, u.uy instead of
0,0.

src/pline.c

@@ -136,7 +136,10 @@ pline_mon(struct monst *mtmp, const char *line, ...)
 {
     va_list the_args;
 
-    set_msg_xy(mtmp->mx, mtmp->my);
+    if (mtmp == &gy.youmonst)
+        set_msg_xy(0, 0);
+    else
+        set_msg_xy(mtmp->mx, mtmp->my);
 
     va_start(the_args, line);
     vpline(line, the_args);