pgsql: Fix contrib/pg_trgm's similarity() function for trigram-free str

Fix contrib/pg_trgm's similarity() function for trigram-free strings.

Cases such as similarity('', '') produced a NaN result due to computing
0/0.  Per discussion, make it return zero instead.

This appears to be the basic cause of bug #7867 from Michele Baravalle,
although it remains unclear why her installation doesn't think Cyrillic
letters are letters.

Back-patch to all active branches.

Branch
------
REL8_4_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/697f5cadf5d81eeee3f3018d711ef33261dc7cd5

Modified Files
--------------
contrib/pg_trgm/expected/pg_trgm.out |    6 ++++++
contrib/pg_trgm/sql/pg_trgm.sql      |    2 ++
contrib/pg_trgm/trgm_op.c            |    8 ++++++--
3 files changed, 14 insertions(+), 2 deletions(-)