contrib/pg_freespacemap first check input argument, then relation_open.

[jian he]

Hi.

In:
https://git.postgresql.org/cgit/postgresql.git/tree/contrib/pg_freespacemap/pg_freespacemap.c

rel = relation_open(relid, AccessShareLock);

if (blkno < 0 || blkno > MaxBlockNumber)
ereport(ERROR,
(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
errmsg("invalid block number")));

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should it first check input arguments, then relation_open?
Does ereport automatically unlock the relation?