Fix compiler warning.

get_user_name returns const char *, but we were assigning the result
to a char * variable.

contrib/pg_upgrade/pg_upgrade.h

@@ -443,7 +443,7 @@ void		check_pghost_envvar(void);
 /* util.c */
 
 char	   *quote_identifier(const char *s);
-int			get_user_info(char **user_name);
+int			get_user_info(char **user_name_p);
 void		check_ok(void);
 void
 report_status(eLogType type, const char *fmt,...)

contrib/pg_upgrade/util.c

@@ -205,9 +205,10 @@ quote_identifier(const char *s)
  * get_user_info()
  */
 int
-get_user_info(char **user_name)
+get_user_info(char **user_name_p)
 {
 	int			user_id;
+	const char *user_name;
 	char	   *errstr;
 
 #ifndef WIN32
@@ -216,12 +217,12 @@ get_user_info(char **user_name)
 	user_id = 1;
 #endif
 
-	*user_name = get_user_name(&errstr);
-	if (!*user_name)
+	user_name = get_user_name(&errstr);
+	if (!user_name)
 		pg_fatal("%s\n", errstr);
 
 	/* make a copy */
-	*user_name = pg_strdup(*user_name);
+	*user_name_p = pg_strdup(user_name);
 
 	return user_id;
 }