fix segfault in atom map hash when using "clear".

src/atom.cpp

@@ -211,7 +211,7 @@ Atom::Atom(LAMMPS *lmp) : Pointers(lmp)
   tag_enable = 1;
   map_style = map_user = 0;
   map_tag_max = -1;
-  map_maxarray = map_nhash = -1;
+  map_maxarray = map_nhash = map_nbucket = -1;
 
   max_same = 0;
   sametag = NULL;

src/atom_map.cpp

@@ -335,7 +335,7 @@ void Atom::map_delete()
       map_bucket = NULL;
       map_hash = NULL;
     }
-    map_nhash = 0;
+    map_nhash = map_nbucket = 0;
   }
 }