Bug#45288: pb2 returns a lot of compilation warnings on linux

Fix warnings related to the use of the deprecated gets() function
and passing NULL to non-pointer argument of the sys_var constructor.

plugin/auth/dialog.c:
  Do not use the deprecated gets() function.
sql/sys_vars.h:
  Do not pass NULL to a non-pointer argument of the sys_var constructor.

plugin/auth/dialog.c

@@ -181,7 +181,7 @@ mysql_declare_plugin_end;
   To support all this variety, the dialog plugin has a callback function
   "authentication_dialog_ask". If the client has a function of this name
   dialog plugin will use it for communication with the user. Otherwise
-  a default gets() based implementation will be used.
+  a default fgets() based implementation will be used.
 */
 
 /**
@@ -208,12 +208,15 @@ static mysql_authentication_dialog_ask_t ask;
 static char *builtin_ask(MYSQL *mysql __attribute__((unused)),
                          int type __attribute__((unused)),
                          const char *prompt,
-                         char *buf, int buf_len __attribute__((unused)))
+                         char *buf, int buf_len)
 {
+  char *ptr;
   fputs(prompt, stdout);
   fputc(' ', stdout);
-  if (gets(buf) == 0)
-    return 0;
+  if (fgets(buf, buf_len, stdin) == NULL)
+    return NULL;
+  if ((ptr= strchr(buf, '\n')))
+    *ptr= 0;
 
   return buf;
 }

sql/sys_vars.h

@@ -460,10 +460,8 @@ public:
           const char *comment, enum charset_enum is_os_charset_arg)
     : sys_var(&all_sys_vars, name_arg, comment,
               sys_var::READONLY+sys_var::ONLY_SESSION, 0, -1,
-              NO_ARG, SHOW_CHAR, (intptr)NULL,
-              0, VARIABLE_NOT_IN_BINLOG,
-              0, 0,
-              0, 0, PARSE_NORMAL)
+              NO_ARG, SHOW_CHAR, 0, NULL, VARIABLE_NOT_IN_BINLOG,
+              NULL, NULL, 0, NULL, PARSE_NORMAL)
   {
     is_os_charset= is_os_charset_arg == IN_FS_CHARSET;
     option.var_type= GET_STR;