Fix some cppcheck warnings.

Despite it is nonsense and cppcheck should understand the code better :-)

src/cryptsetup_reencrypt.c

@@ -303,7 +303,11 @@ static int create_empty_header(const char *new_file, const char *old_file,
 
 	log_dbg("Creating empty file %s of size %lu.", new_file, (unsigned long)size);
 
-	if (!size || !(buf = malloc(size)))
+	if (!size)
+		return -EINVAL;
+
+	buf = malloc(size);
+	if (!buf)
 		return -ENOMEM;
 	memset(buf, 0, size);