intmap: add test case which failed, extracted from real world usage.

Because intmap_after_() would simply examine the critbits to walk the
tree, it wouldn't realize that it might be in the completely wrong tree.

In this case:

         Bit 4:
         0   1
        /     \
       /       \
  100000011  100001011

When we ask for intmap_after_(011111111) we would check the critbit, it's
a 1, so we end up on the right leaf instead of the left.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

ccan/intmap/test/run-after-fail.c

+#include <ccan/intmap/intmap.h>
+#include <ccan/intmap/intmap.c>
+#include <ccan/tap/tap.h>
+
+int main(void)
+{
+	UINTMAP(const char *) map;
+	u64 idx;
+
+	/* This is how many tests you plan to run */
+	plan_tests(2);
+
+	uintmap_init(&map);
+	assert(uintmap_add(&map, 0x103, "103"));
+	assert(uintmap_add(&map, 0x10b, "10b"));
+
+	uintmap_first(&map, &idx);
+	ok1(idx > 0xF);
+	idx = 0xF;
+	ok1(strcmp(uintmap_after(&map, &idx), "103") == 0);
+
+	uintmap_clear(&map);
+
+	/* This exits depending on whether all tests passed */
+	return exit_status();
+}