pgsql: Fix InitializeSessionUserId not to deference NULL rolename point

Fix InitializeSessionUserId not to deference NULL rolename pointer.

Dmitriy Sarafannikov, reviewed by Michael Paquier and Haribabu Kommi,
with a minor fix by me.

Branch
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REL9_5_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/7963731f94fd57293a651aa2a9a2e31dd11d6541

Modified Files
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src/backend/utils/init/miscinit.c | 24 +++++++++++++++++-------
1 file changed, 17 insertions(+), 7 deletions(-)