fix "couldn't place lregion type 5"

Implement a fix for the problem From a bug report:
if the destination position on the Plane of Fire has a randomly
placed trap on it, you'd get an impossibility warning of "couldn't
place lregion type 5" (and then arrive successfully at the target
spot anyway).  As his investigation indicated, the code to remove
such traps wasn't being reached because the `bad_location' check
yields true for trapped spots.

src/mkmaze.c

@@ -1,4 +1,4 @@
-/*	SCCS Id: @(#)mkmaze.c	3.4	2002/03/12	*/
+/*	SCCS Id: @(#)mkmaze.c	3.4	2002/04/04	*/
 /* Copyright (c) Stichting Mathematisch Centrum, Amsterdam, 1985. */
 /* NetHack may be freely redistributed.  See license for details. */
 
@@ -288,13 +288,18 @@ xchar rtype;
 boolean oneshot;
 d_level *lev;
 {
-    if (bad_location(x, y, nlx, nly, nhx, nhy)) return FALSE;
-    if (oneshot) {
-	/* must make due with the only location possible */
-	/* avoid failure due to a misplaced trap */
-	/* it might still fail if there's a dungeon feature here */
-	struct trap *t = t_at(x,y);
-	if (t) deltrap(t);
+    if (bad_location(x, y, nlx, nly, nhx, nhy)) {
+	if (!oneshot) {
+	    return FALSE;		/* caller should try again */
+	} else {
+	    /* Must make due with the only location possible;
+	       avoid failure due to a misplaced trap.
+	       It might still fail if there's a dungeon feature here. */
+	    struct trap *t = t_at(x,y);
+
+	    if (t && t->ttyp != MAGIC_PORTAL) deltrap(t);
+	    if (bad_location(x, y, nlx, nly, nhx, nhy)) return FALSE;
+	}
     }
     switch (rtype) {
     case LR_TELE: