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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REL9_6_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/88e1e91da58422c5cf0e6e2d32f2aa15d75b8de9

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