pg_amcheck: Fix inconsistency in memory freeing

pg_amcheck: Fix inconsistency in memory freeing

The function in charge of freeing the memory from a result created by
PQescapeIdentifier() has to be PQfreemem(), to ensure that both
allocation and free come from libpq, but one spot in pg_amcheck was
missing that.

Oversight in b859d94c6389.

Author: Ranier Vilela <ranier.vf@gmail.com>
Reviewed-by: vignesh C <vignesh21@gmail.com>
Discussion: http://postgr.es/m/CAEudQArD_nKSnYCNUZiPPsJ2tNXgRmLbXGSOrH1vpOF_XtP0Vg@mail.gmail.com
Discussion: http://postgr.es/m/CAEudQArbTWVSbxq608GRmXJjnNSQ0B6R7CSffNnj2hPWMUsRNg@mail.gmail.com
Backpatch-through: 14