Use realpath() safely (#764369)

realpath(3) manual page says:

    BUGS
        The POSIX.1-2001 standard version of this function is broken by
        design, since it is impossible to determine a suitable size for
        the output buffer, resolved_path.  According to POSIX.1-2001 a
        buffer of size PATH_MAX suffices, but PATH_MAX need not be a
        defined constant, and may have to be obtained using pathconf(3).
        And asking pathconf(3) does not really help, since, on the one
        hand POSIX warns that the result of pathconf(3) may be huge and
        unsuitable for mallocing memory, and on the other hand
        pathconf(3) may return -1 to signify that PATH_MAX is not
        bounded.  The resolved_path == NULL feature, not standardized in
        POSIX.1-2001, but standardized in POSIX.1-2008, allows this
        design problem to be avoided.

The resolved_path == NULL feature of realpath() has existed as a Glibc
extension since realpath() was first added to Glibc 1.90, released in
June 1996.  Therefore it can be used unconditionally.

    https://sourceware.org/git/?p=glibc.git;a=commitdiff;h=fa0bc87c32d02cd81ec4d0ae00e0d943c683e6e1

Bug 764369 - Use realpath() safely