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_1_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/fb6825fe57651dfd4dfe9d2041c715b03c5c6323

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