rustls: make read_file_into not reject good files

rustls: make read_file_into not reject good files

For files with sizes using an exact multiple of 256 bytes, the final
successful read(s) filled the buffer(s) and the subsequent fread
returned 0 for EOF, which caused read_file_into to fail.

Now, it needs to return 0 and not be EOF to be an error.

Follow-up to dd95a49d493d55db38b352fdbda2

Pointed out by ZeroPath
Closes #19104