rand/rand.c (rand_getgood): Stretch the output buffer if necessary.

rand/rand.c (rand_getgood): Stretch the output buffer if necessary.

It's possible to have `r->o == RAND_BUFSZ' in the main loop, while
`r->obits' is larger than the requested size.  The following program
contrives this situation, though it can (and does) happen organically.

	#include <stdio.h>
	#include <stdlib.h>

	#include "noise.h"
	#include "rand.h"

	int main(void)
	{
	  rand_pool pool;
	  unsigned char buf[64];
	  size_t n;

	  rand_init(&pool);
	  rand_noisesrc(&pool, &noise_source);
	  rand_seed(&pool, 64);

	  while (pool.obits < RAND_OBITS) rand_seed(&pool, RAND_IBITS);
	  while (pool.o < RAND_BUFSZ) {
	    n = RAND_BUFSZ - pool.o; if (n > sizeof(buf)) n = sizeof(buf);
	    rand_getgood(&pool, buf, n);
	  }
	  rand_getgood(&pool, buf, 4);
	  return (0);
	}

When this happens, `rand_getgood' gets stuck in an infinite loop,
trimming the chunk size to zero because the output buffer is exhausted,
but not refilling it because there's still notional entropy remaining.
Detect this situation and stretch the output buffer when there's nothing
left, as in `rand_get'.