add test for zero-sized alloc and aligned alloc

test/integration/malloc.c

@@ -0,0 +1,16 @@
+#include "test/jemalloc_test.h"
+
+TEST_BEGIN(test_zero_alloc) {
+	void *res = malloc(0);
+	assert(res);
+	size_t usable = malloc_usable_size(res);
+	assert(usable > 0);
+	free(res);
+}
+TEST_END
+
+int
+main(void) {
+	return test(
+	    test_zero_alloc);
+}