4265. [bug] Address unchecked isc_mem_get calls. [RT #41187]

(cherry picked from commit 8e73941f33)

lib/isc/md5.c

@@ -57,6 +57,8 @@ isc_md5_invalidate(isc_md5_t *ctx) {
 
 void
 isc_md5_update(isc_md5_t *ctx, const unsigned char *buf, unsigned int len) {
+	if (len == 0U)
+		return;
 	RUNTIME_CHECK(EVP_DigestUpdate(ctx,
 				       (const void *) buf,
 				       (size_t) len) == 1);