memory tracking fix

'heaputil' is producing a lot of complaints.  This fixes one of them,
about freeing memory that was never allocated.  In this case, it's
when removing an overview annotation for a level.  The annotation
is using dupstr_n() and not being recorded due to dupstr_n() being
placed after MONITOR_HEAP undefines the macro that overrides alloc().

There's only one use of dupstr_n(), and its length checking isn't
needed there, so just switch to dupstr() and comment out the
implementation of dupstr_n().  I left the prototype in extern.h;
that's harmless.

If dupstr_n() needs to be resurrected, a second MONITOR_HEAP-aware
version should be implemented, with corresponding macro to choose
which one to use.

src/dungeon.c

@@ -2554,7 +2554,9 @@ query_annotation(d_level *lev)
     /* add new annotation, unless it's all spaces (which will be an
        empty string after mungspaces() above) */
     if (*nbuf && strcmp(nbuf, " ")) {
-        mptr->custom = dupstr_n(nbuf,&mptr->custom_lth);
+        mptr->custom = dupstr(nbuf);
+        /* _lth field does not include trailing '\0' in the count */
+        mptr->custom_lth = (unsigned) strlen(mptr->custom);
     }
 }
 
@@ -2668,27 +2670,24 @@ rm_mapseen(int ledger_num)
         if (svd.dungeons[mptr->lev.dnum].ledger_start + mptr->lev.dlevel
             == ledger_num)
             break;
-
     if (!mptr)
         return;
 
     if (mptr->custom)
-        free((genericptr_t) mptr->custom);
+        free((genericptr_t) mptr->custom), mptr->custom = (char *) NULL;
 
-    bp = mptr->final_resting_place;
-    while (bp) {
+    bpnext = mptr->final_resting_place;
+    while ((bp = bpnext) != NULL) {
         bpnext = bp->next;
-        free(bp);
-        bp = bpnext;
+        free((genericptr_t) bp);
     }
 
     if (mprev) {
         mprev->next = mptr->next;
-        free(mptr);
     } else {
         svm.mapseenchn = mptr->next;
-        free(mptr);
     }
+    free(mptr);
 }
 
 staticfn void