random_seed: allow to block on /dev/random

If both /dev/random and /dev/urandom failed to return data, an
uninitialised value might be returned.  Since most systems have a
non-blocking /dev/urandom or have /dev/random with similar properties,
the chance of blocking is minimal, and the alternative of returning
non-random data is worse.

Originally committed as revision 23930 to svn://svn.ffmpeg.org/ffmpeg/trunk
