[PATCH] Fix potential memoryleak in guc.c

Hi all

In src\backend\utils\misc\guc.c, I found a potential memory leak.

make_absolute_path() return a malloc'd copy, we should free memory before the function return false.
----------------------------------------------------------------------------
SelectConfigFiles(const char *userDoption, const char *progname)
{
......
    /* configdir is -D option, or $PGDATA if no -D */
    if (userDoption)
        configdir = make_absolute_path(userDoption);  ★
    else
        configdir = make_absolute_path(getenv("PGDATA")); ★

    if (configdir && stat(configdir, &stat_buf) != 0)
    {
        write_stderr("%s: could not access directory \"%s\": %s\n",
                     progname,
                     configdir,
                     strerror(errno));
        if (errno == ENOENT)
            write_stderr("Run initdb or pg_basebackup to initialize a PostgreSQL data directory.\n");
        ★// Need to free memory of configdir
        return false;
    }
......
---------------------------------------------------------------------------

Refer to the following files for the implementation of make_absolute_path().

file:  src\port\path.c
/*
 * make_absolute_path
 *
 * If the given pathname isn't already absolute, make it so, interpreting
 * it relative to the current working directory.
 *
 * Also canonicalizes the path.  The result is always a malloc'd copy.