Test uuid for NULL

Add test for NULL before passing uuid as src argument to memcpy.
As memcpy function is declared as function not accepting NULL.
Though we pass NULL only with zero length so this patch presents
no functional change to the code.

tools/dmsetup.c

@@ -343,7 +343,9 @@ static char *_extract_uuid_prefix(const char *uuid, const int separator)
 		return NULL;
 	}
 
-	memcpy(uuid_prefix, uuid, len);
+	if (uuid)
+		memcpy(uuid_prefix, uuid, len);
+
 	uuid_prefix[len] = '\0';
 
 	return uuid_prefix;