Tweaked test/bench prng to convert 0 -> -1

Like many prngs, xorshift breaks down when the internal state is 0. The
common fix is to explicitly check for this and replace with a 1 when
this happens (usually when seeding, in this API we have to check every
update, this is less efficient but I don't think we really care).

As a slight tweak, this now checks for 0 but replaces it with -1. This
makes seed=0 different from seed=1, which is nice when using
seed=range(0,n) in tests/benches.

runners/bench_runner.c

@@ -587,6 +587,11 @@ uint32_t bench_prng(uint32_t *state) {
     // A simple xorshift32 generator, easily reproducible. Keep in mind
     // determinism is much more important than actual randomness here.
     uint32_t x = *state;
+    // must be non-zero, use uintmax here so that seed=0 is different
+    // from seed=1 and seed=range(0,n) makes a bit more sense
+    if (x == 0) {
+        x = -1;
+    }
     x ^= x << 13;
     x ^= x >> 17;
     x ^= x << 5;

runners/test_runner.c

@@ -603,6 +603,11 @@ uint32_t test_prng(uint32_t *state) {
     // A simple xorshift32 generator, easily reproducible. Keep in mind
     // determinism is much more important than actual randomness here.
     uint32_t x = *state;
+    // must be non-zero, use uintmax here so that seed=0 is different
+    // from seed=1 and seed=range(0,n) makes a bit more sense
+    if (x == 0) {
+        x = -1;
+    }
     x ^= x << 13;
     x ^= x >> 17;
     x ^= x << 5;