Fix stack-use-after-scope in dns_message_checksig test

Previously stack with buffer for test dns message went out of scope
before the message was processed. For fuzz testing its better to avoid
allocation, so let's avoid allocations completely and use simplest
possible static buffer.

Fixes: #3565
(cherry picked from commit 16377100ae)

fuzz/dns_message_checksig.c

@@ -213,7 +213,7 @@ create_message(dns_message_t **messagep, const uint8_t *data, size_t size,
 	isc_result_t result;
 	dns_message_t *message = NULL;
 	isc_buffer_t b;
-	unsigned char buf[65535];
+	static unsigned char buf[65535];
 
 	isc_buffer_init(&b, buf, sizeof(buf));