Fix crash that can occur in decode_mixed.

If a player names an object with a name that ends in '\\', drops
that object on the floor nearby and does a look at nearby objects,
then the game will crash.  This is caused by stack corruption when
the decode loop skips over the decode string terminator.

src/mapglyph.c

@@ -313,6 +313,15 @@ const char *str;
 #endif
             case '\\':
                 break;
+            case '\0':
+                /* String ended with '\\'.  This can happen when someone
+                   names an object with a name ending with '\\', drops the
+                   named object on the floor nearby and does a look at all
+                   nearby objects. */
+                /* brh - should we perhaps not allow things to have names
+                   that contain '\\' */
+                str = save_str;
+                break;
             }
         }
         *put++ = *str++;