Avoid unncessary always true test (src/backend/storage/buffer/bufmgr.c)

Hi,

Per Coverity.

At function ExtendBufferedRelShared, has a always true test.
eb.rel was dereferenced one line above, so in
if (eb.rel) is always true.

I think it's worth removing the test, because Coverity raises dozens of
alerts thinking eb.rel might be NULL.
Besides, one less test is one less branch.

regards,
Ranier Vilela