avoid newsym(0,0) for vault guard

When a vault guard is being moving off the map to <0,0> to wait until
his temporary corridor gets removed, don't try to update the map for
that off-the-screen location in order to avoid triggering impossible()
from newsym().

Plus a trivial tweak to NH_abort().  Its argument is never modified so
declare it as such.

doc/fixes5-0-1.txt

@@ -12,8 +12,10 @@ fix issue where the new game startup message was stating 'Work-in-progress'
 	even when NH_DEVEL_STATUS was set to NH_STATUS_POSTRELEASE
 if 'mention_decor' was set in config file or NETHACKOPTIONS,
 	starting the game told you that "you are standing on stairs which lead
-	out of the dungeon", even when the player entered the tutorial where that
-	wasn't true
+	out of the dungeon", even when the player entered the tutorial where
+	that wasn't true
+when a vault guard was moved off the map, an impossible about trying to update
+	map location <0,0> was issued
 doc: use full URL for nethack.org in Guidebook.mn
 doc: fix a paste error in Guidebook.tex
 doc: fix spelling correction in nethack.6

include/extern.h

@@ -995,7 +995,7 @@ extern void done1(int);
 extern int done2(void);
 extern void done_in_by(struct monst *, int) NONNULLARG1;
 extern void done_object_cleanup(void);
-extern void NH_abort(char *);
+extern void NH_abort(const char *);
 #endif /* !MAKEDEFS_C && MDLIB_C && !CPPREGEX_C */
 #if !defined(CPPREGEX_C)
 ATTRNORETURN extern void panic(const char *, ...) PRINTF_F(1, 2) NORETURN;

src/end.c

@@ -1896,7 +1896,7 @@ build_english_list(char *in)
 # endif
 
 void
-NH_abort(char *why USED_FOR_CRASHREPORT)
+NH_abort(const char *why USED_FOR_CRASHREPORT)
 {
 #ifdef PANICTRACE
     int gdb_prio = SYSOPT_PANICTRACE_GDB;
@@ -1912,7 +1912,7 @@ NH_abort(char *why USED_FOR_CRASHREPORT)
 
 #ifdef PANICTRACE
 #ifdef CRASHREPORT
-    if(!submit_web_report(1, "Panic", why))
+    if (!submit_web_report(1, "Panic", why))
 #endif
     {
 #ifndef VMS

src/monmove.c

@@ -1653,7 +1653,9 @@ postmov(
             u_on_newpos(mtmp->mx, mtmp->my);
             swallowed(0);
         } else {
-            newsym(mtmp->mx, mtmp->my);
+            /* only call newsym() when not a vault guard moving to <0,0> */
+            if (mtmp->mx)
+                newsym(mtmp->mx, mtmp->my);
         }
     } /* mmoved==MMOVE_MOVED */