nbd: Avoid off-by-one in long export name truncation

When snprintf returns the same value as the buffer size, the final
byte was truncated to ensure a NUL terminator.  Fortunately, such long
export names are unusual enough, with no real impact other than what
is displayed to the user.

Fixes: 5c86bdf
Reported-by: Max Reitz <[email protected]>
Signed-off-by: Eric Blake <[email protected]>
Message-Id: <[email protected]>
Reviewed-by: Vladimir Sementsov-Ogievskiy <[email protected]>

Author: ebblake

block/nbd.c

@@ -2002,7 +2002,7 @@ static void nbd_refresh_filename(BlockDriverState *bs)
         len = snprintf(bs->exact_filename, sizeof(bs->exact_filename),
                        "nbd://%s:%s", host, port);
     }
-    if (len > sizeof(bs->exact_filename)) {
+    if (len >= sizeof(bs->exact_filename)) {
         /* Name is too long to represent exactly, so leave it empty. */
         bs->exact_filename[0] = '\0';
     }