Prevent overflow for block number in buffile.c

Prevent overflow for block number in buffile.c

As coded, the start block calculated by BufFileAppend() would overflow
once more than 16k files are used with a default block size.  This issue
existed before b1e5c9fa9ac4, but there's no reason not to be clean about
it.

Per report from Coverity, with a fix suggested by Tom Lane.