Source: Avoid gcc 12 compilation warning

GCC 12 warns:

    warning: '%04d' directive output may be truncated writing between
    4 and 11 bytes into a region of size 5 [-Wformat-truncation=]

The surrounding logic guarantees the formatted integer will never
be more than 4 bytes, but it doesn't hurt to use a larger buffer.
This GCC behavior is documented:

    https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104012#c5