Fix if/while thinko in read_stream.c edge case.

Fix if/while thinko in read_stream.c edge case.

When we determine that a wanted block can't be combined with the current
pending read, it's time to start that read to get it out of the way.  An
"if" in that code path should have been a "while", because it might take
more than one go in case of partial reads.  This was only broken for
smaller ranges, as the more common case of io_combine_limit-sized ranges
is handled earlier in the code and knows how to loop, hiding the bug for
a while.

Discovered while testing large parallel sequential scans of partially
cached tables.  The ramp-up-and-down block allocator for parallel scans
could hit the problem case and skip some blocks near the end that should
have been streamed.

Defect in commit b5a9b18c.

Discussion: https://postgr.es/m/CA%2BhUKG%2Bh8Whpv0YsJqjMVkjYX%2B80fTVc6oi-V%2BzxJvykLpLHYQ%40mail.gmail.com