pgsql: Fix detection of unfinished Unicode surrogate pair at end of str

Fix detection of unfinished Unicode surrogate pair at end of string.

The U&'...' and U&"..." syntaxes silently discarded a surrogate pair
start (that is, a code between U+D800 and U+DBFF) if it occurred at
the very end of the string. This seems like an obvious oversight,
since we throw an error for every other invalid combination of surrogate
characters, including the very same situation in E'...' syntax.

This has been wrong since the pair processing was added (in 9.0),
so back-patch to all supported branches.

Discussion: https://postgr.es/m/19113.1482337898@sss.pgh.pa.us

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/a8ae12322ae056ebbe9d83cf16b4c92e86a6ac28

Modified Files
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src/backend/parser/scan.l | 7 +++++++
1 file changed, 7 insertions(+)