worm display bug

This fixes a long worm display bug.  The bug wasn't really in the worm code, it
was in goodpos().  goodpos() could place worm segments on top of one another.
If you split a worm when the tail was located on top of the head, the tail
would be removed from the screen, making the head seem to vanish.

doc/fixes34.1

@@ -95,6 +95,8 @@ corpses in bones files don't retain their role characteristic
 boulder was not displayed if blind and discovered with a monster known via
 	ESP behind it
 don't claim that statue comes to life if the monster it turns into is invisible
+fix goodpos() so worm segments don't get placed on top of each other (causing
+	a possible display problem if the worm is cut in two)
 
 
 Platform- and/or Interface-Specific Fixes

src/teleport.c

@@ -42,7 +42,17 @@ struct monst *mtmp;
 	if (mtmp) {
 	    struct monst *mtmp2 = m_at(x,y);
 
-	    if (mtmp2 && mtmp2 != mtmp)
+	    /* Be careful with long worms.  A monster may be placed back in
+	     * its own location.  Normally, if m_at() returns the same monster
+	     * that we're trying to place, the monster is being placed in its
+	     * own location.  However, that is not correct for worm segments,
+	     * because all the segments of the worm return the same m_at().
+	     * Actually we overdo the check a little bit--a worm can't be placed
+	     * in its own location, period.  If we just checked for mtmp->mx
+	     * != x || mtmp->my != y, we'd miss the case where we're called
+	     * to place the worm segment and the worm's head is at x,y.
+	     */
+	    if (mtmp2 && (mtmp2 != mtmp || mtmp->wormno))
 		return FALSE;
 
 	    mdat = mtmp->data;