Fix behaviour of fetch() without any data with PostgreSQL

[vadz]

If execute() didn't bring any data from the server, all statement member variables remained uninitialized, resulting in problems (UB, infinite loops, ...) if fetch() is called later.

Fix this by initializing numberOfRows_ and checking for it in fetch() and simply not doing anything there if it is 0.

Closes #1328.

[GregTheMadMonk]

Yes, the random number reads in fetch are (or at least seem to be :) ) fixed by this. Thank you!

normally we shouldn't be even called in this case

How should I have approached writing a "generic" interface for queries with SOCI, e.g. an interactive tool to execute requests and view their results without knowing in advance if the query returns any data or not? Or was this indeed the only way to do it without parsing the requests manually beforehand?

P.S.

From #1329:
I don't see a sanitizer error here. I do see a failed

 CHECK( fetch_all("UPDATE soci_test SET flag=TRUE", flag) == 0 )
due to unexpected exception with message:
 Null value fetched and no indicator defined for the parameter number 1 while
 fetching data from "UPDATE soci_test SET flag=TRUE".
I'm not sure why are you not seeing this, as it seems correct to me.

I'm... not really sure. I guess that might be just UB justifying the "undefined" in its name, because even with everything but that query commented I still get

$ grep -n -A9 'Reads several rows - fine' ../tests/postgresql/test-postgresql.cpp && ninja && ./bin/soci_postgresql_test 'host=localhost port=5432 dbname=test user=postgres password='
1449:        // Reads several rows - fine
1450-        // CHECK(fetch_all("SELECT * FROM soci_test", flag) == 2);
1451-        // Reads zero rows - fine
1452-        // CHECK(fetch_all("SELECT * FROM soci_test WHERE flag = NOT flag", flag) == 0);
1453-        // UPDATE causes a sanitizer error
1454-        CHECK(fetch_all("UPDATE soci_test SET flag=TRUE", flag) == 0);
1455-        // INSERT causes a sanitizer error
1456-        // CHECK(fetch_all("INSERT INTO soci_test(flag) VALUES (FALSE)", flag) == 0);
1457-        // DELETE causes a sanitizer error
1458-        // CHECK(fetch_all("DELETE FROM soci_test WHERE NOT flag", flag) == 0);
--
1460:        // Reads several rows - fine
1461-        // CHECK(fetch_all("SELECT * FROM soci_test", row) == 2);
1462-        // Reads zero rows - fine
1463-        // CHECK(fetch_all("SELECT * FROM soci_test WHERE flag = NOT flag", row) == 0);
1464-        // UPDATE causes a sanitizer error
1465-        // CHECK(fetch_all("UPDATE soci_test SET flag=TRUE", row) == 0);
1466-        // INSERT causes a sanitizer error
1467-        // CHECK(fetch_all("INSERT INTO soci_test(flag) VALUES (FALSE)", row) == 0);
1468-        // DELETE causes a sanitizer error
1469-        // CHECK(fetch_all("DELETE FROM soci_test WHERE NOT flag", row) == 0);
[2/2] Linking CXX executable bin/soci_postgresql_test

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
soci_postgresql_test is a Catch v2.13.10 host application.
Run with -? for options

-------------------------------------------------------------------------------
PostgreSQL ROWID
-------------------------------------------------------------------------------
/media/ssd/projects/cpp/soci/psql-statement-ub/tests/postgresql/test-postgresql.cpp:154
...............................................................................

/media/ssd/projects/cpp/soci/psql-statement-ub/tests/postgresql/test-postgresql.cpp:162: warning:
  Skipping test because OIDs are no longer supported in PostgreSQL 180001

NOTICE:  table "soci_test" does not exist, skipping
NOTICE:  table "soci_test" does not exist, skipping
/media/ssd/projects/cpp/soci/psql-statement-ub/src/backends/postgresql/statement.cpp:568:17: runtime error: signed integer overflow: -1094795586 + -1094795586 cannot be represented in type 'int'
===============================================================================
All tests passed (2425 assertions in 96 test cases)

I wanted to try with the clang's memory sanitizer because I expected the UB sanitizer to report an undefined memory read and... it threw a use-of-uninitialized-value from Catch.hpp:12542:21 at me even before it could get to the test :| Which is weird, could this be a false-positive?