Fix for the bug #7344 (multiple server_init/server_end lead to crash)

Some variable wasn't moved to the initial state during mysql_server_end


myisam/ft_stopwords.c:
  resetting ft_stopword_file
sql/mysqld.cc:
  free_charsets() call added

myisam/ft_stopwords.c

@@ -125,4 +125,5 @@ void ft_free_stopwords()
     my_free((char*) stopwords3,MYF(0));
     stopwords3=0;
   }
+  ft_stopword_file= 0;
 }

sql/mysqld.cc

@@ -950,6 +950,7 @@ void clean_up(bool print_message)
   item_user_lock_free();
   lex_free();				/* Free some memory */
   set_var_free();
+  free_charsets();
 #ifdef HAVE_DLOPEN
   if (!opt_noacl)
     udf_free();