[libc] Always use a non-zero seed for the (non-crypto) RNG

The non-cryptographic RNG implemented by random() has the property
that a seed value of zero will result in a generated sequence of
all-zero values.  This situation can arise if currticks() returns zero
at start of day.

Work around this problem by falling back to a fixed non-zero seed if
necessary.

This has no effect on the separate DRBG used by cryptographic code.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/core/random.c

  */
 void srandom ( unsigned int seed ) {
 	rnd_seed = seed;
+	if ( ! rnd_seed )
+		rnd_seed = 4; /* Chosen by fair dice roll */
 }
 
 /**