test: add tests for zero sized realloc(3)

C23 declared calling realloc(3) with a non-NULL pointer and zero size
Undefined behavior.
Check that hardened_malloc handles that case sanely by free'ing the old
pointer and returning a special pointer, like `malloc(3)` called with
size zero.

test/realloc_c23_undefined_behaviour.c

@@ -0,0 +1,19 @@
+#include <stdio.h>
+#include <stdlib.h>
+
+#include "test_util.h"
+
+OPTNONE int main(void) {
+    char *p, *q, *r;
+
+    p = malloc(16);
+    if (!p) {
+        return 1;
+    }
+
+    q = realloc(p, 0);
+
+    free(q);
+
+    return 0;
+}