pgsql: In PQsendQueryStart(), avoid leaking any left-over async result.

In PQsendQueryStart(), avoid leaking any left-over async result.

Ordinarily there would not be an async result sitting around at this
point, but it appears that in corner cases there can be.  Considering
all the work we're about to launch, it's hardly going to cost anything
noticeable to check.

It's been like this forever, so back-patch to all supported branches.

Report: <CAD-Qf1eLUtBOTPXyFQGW-4eEsop31tVVdZPu4kL9pbQ6tJPO8g@mail.gmail.com>

Branch
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REL9_5_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/4f87f7640981bae3a37e14bae744452e80e9545d

Modified Files
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src/interfaces/libpq/fe-exec.c | 3 +--
1 file changed, 1 insertion(+), 2 deletions(-)