Added another examples for Chapter 9

chp9/test_malloc.c

@@ -0,0 +1,16 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <malloc.h>
+
+int main(void) {
+	char *buf = (char *)malloc(256);
+	if (!buf) {
+		perror("malloc");
+		return 1;
+	}
+
+	size_t size = malloc_usable_size(buf);
+	printf("%d\n", size);
+	free(buf);
+	return 0;
+}