pgsql: Fix a problem with parallel workers being unable to restore role

Fix a problem with parallel workers being unable to restore role.

check_role() tries to verify that the user has permission to become the
requested role, but this is inappropriate in a parallel worker, which
needs to exactly recreate the master's authorization settings.  So skip
the check in that case.

This fixes a bug in commit 924bcf4f16d54c55310b28f77686608684734f42.

Branch
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REL9_5_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/73d71cde5751e06d372431178e740835284eb132

Modified Files
--------------
src/backend/access/transam/parallel.c |    7 +++++++
src/backend/commands/variable.c       |    8 ++++++--
src/include/access/parallel.h         |    1 +
3 files changed, 14 insertions(+), 2 deletions(-)