MDEV-32862 MYSQL struct in C/C and server differs

move MYSQL::fields down, replacing MYSQL::unused5

this way only MYSQL::fields and MYSQL::field_alloc will still have
different offset in C/C and the server, but all other MYSQL members
will get back in sync.

luckily, plugins shouldn't need MYSQL::fields or MYSQL::field_alloc

added a check to ensure both MYSQL structures are always of
the same size.

include/mysql.h

@@ -274,7 +274,6 @@ typedef struct st_mysql
   char		*host,*user,*passwd,*unix_socket,*server_version,*host_info;
   char          *info, *db;
   const struct charset_info_st *charset;
-  MYSQL_FIELD	*fields;
   MEM_ROOT	field_alloc;
   my_ulonglong affected_rows;
   my_ulonglong insert_id;		/* id if insert on table with NEXTNR */
@@ -296,7 +295,8 @@ typedef struct st_mysql
   /* session-wide random string */
   char	        scramble[SCRAMBLE_LENGTH+1];
   my_bool       auto_local_infile;
-  void *unused2, *unused3, *unused4, *unused5;
+  void *unused2, *unused3, *unused4;
+  MYSQL_FIELD	*fields;
 
   LIST  *stmts;                     /* list of all statements */
   const struct st_mysql_methods *methods;

tests/mysql_client_fw.c

@@ -1426,6 +1426,14 @@ int main(int argc, char **argv)
     tests_to_run[i]= NULL;
   }
 
+#ifdef _WIN32
+  /* must be the same in C/C and embedded, 1208 on 64bit, 968 on 32bit */
+  compile_time_assert(sizeof(MYSQL) == 60*sizeof(void*)+728);
+#else
+  /* must be the same in C/C and embedded, 1272 on 64bit, 964 on 32bit */
+  compile_time_assert(sizeof(MYSQL) == 77*sizeof(void*)+656);
+#endif
+
   if (mysql_server_init(embedded_server_arg_count,
                         embedded_server_args,
                         (char**) embedded_server_groups))