Add Sedgewick min NULL fail as regression test

Author: sim642

tests/regression/27-inv_invariants/16-sedgewick.c

@@ -0,0 +1,26 @@
+#include <stddef.h>
+#include <assert.h>
+
+struct node {
+  struct node *left;
+  struct node *right;
+  int key;
+};
+
+// https://old.reddit.com/r/programminghorror/comments/jgrpcu/on_sedgewicks_original_presentation_for_llrb_trees/
+struct node* min(struct node *root) {
+  struct node *x = root;
+  while (x != NULL)
+    x = x->left;
+  if (x == NULL) // WARN (dead branch)
+    return NULL;
+  else
+    return x;
+}
+
+int main() {
+  struct node *root;
+  struct node *m = min(root);
+  assert(m == NULL);
+  return 0;
+}