pgsql: Fix incorrect error message in xlogreader.c

Fix incorrect error message in xlogreader.c

When neither BKPIMAGE_HAS_HOLE nor BKPIMAGE_COMPRESSED is set,
DecodeXLogRecord() checks that bimg_len needs to be equal to BLCKSZ.
However, the associated error message reported the data length
associated to a block (DecodedBkpBlock.data_len), and not the length of
the block (DecodedBkpBlock.bimg_len).

Oversight in 57aa5b2bb11a, probably due to some copy-paste from the
surroundings.

Author: Ayush Tiwari <ayushtiwari(dot)slg01(at)gmail(dot)com>
Discussion: https://postgr.es/m/CAJTYsWVz9ymE7aMt2ZiQvs1bxNvhjfTf+XaJEsdFw7MT5dvtYQ@mail.gmail.com
Backpatch-through: 14

Branch
------
REL_14_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/4305b32c98714773b4545b4827a6b927f5f2e5a5

Modified Files
--------------
src/backend/access/transam/xlogreader.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)