894: Fix error-reporting crash in non-blocking connection.

Fixes: #894

If construction of a non-blocking `connection` failed for a reason
_other_ than an out-of-memory error, then reporting of the error
would fail.  This happened because the error-reporting code tried to
read the error message _after it had already cleared the connection
pointer._  This means reading deallocated memory.

NEWS

@@ -4,6 +4,7 @@
  - Don't call`/bin/true`; macOS doesn't have it.  Just call `true`. (#885)
  - Deprecate `exec_prepared()`; just use `exec()` with a `pqxx::prepped`.
  - Deprecate `exec1()` etc.  Use `result::expect_rows()` etc.
+ - Fix error-reporting crash in non-blocking connection constructor. (#894)
 7.9.2
  - Fix CMake documentation install. (#848)
  - More CMake build fix. (#869)

src/connection.cxx

@@ -115,9 +115,10 @@ pqxx::connection::connection(
     throw std::bad_alloc{};
   if (status() == CONNECTION_BAD)
   {
+    std::string const msg{PQerrorMessage(m_conn)};
     PQfinish(m_conn);
     m_conn = nullptr;
-    throw pqxx::broken_connection{PQerrorMessage(m_conn)};
+    throw pqxx::broken_connection{msg};
   }
 }