pgsql: Loop when necessary in contrib/pgcrypto's pktreader_pull().

Loop when necessary in contrib/pgcrypto's pktreader_pull().

This fixes a scenario in which pgp_sym_decrypt() failed with "Wrong key
or corrupt data" on messages whose length is 6 less than a power of 2.

Per bug #11905 from Connor Penhale.  Fix by Marko Tiikkaja, regression
test case from Jeff Janes.

Branch
------
REL9_3_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/419de696a76f5884e26ecd0905a084b0f57afc93

Modified Files
--------------
contrib/pgcrypto/expected/pgp-decrypt.out |    8 ++++++++
contrib/pgcrypto/pgp-decrypt.c            |    2 +-
contrib/pgcrypto/sql/pgp-decrypt.sql      |    4 ++++
3 files changed, 13 insertions(+), 1 deletion(-)