pgsql: Fix corner case bug in numeric to_char().

Fix corner case bug in numeric to_char().

Trailing-zero stripping applied by the FM specifier could strip zeroes
to the left of the decimal point, for a format with no digit positions
after the decimal point (such as "FM999.").

Reported and diagnosed by Marti Raudsepp, though I didn't use his patch.

Branch
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REL8_4_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/2ab199b354135f199a895c4ee61c521b03a4d321

Modified Files
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src/backend/utils/adt/formatting.c    |   27 ++++++++++++++++++++-------
src/test/regress/expected/numeric.out |   18 ++++++++++++++++++
src/test/regress/sql/numeric.sql      |    4 ++++
3 files changed, 42 insertions(+), 7 deletions(-)