Fix regression with statement re-execution when using Firebird backend.

Since the introduction of endOfRowSet_ flag in 4c3cb62, executing the same
statement again stopped working as the flag was never reset after fetching all
the results of the first execution.

Fix this by resetting it in firebird_statement_backend::execute().

Also add a test verifying that this does indeed work.

src/backends/firebird/statement.cpp

@@ -438,6 +438,10 @@ firebird_statement_backend::execute(int number)
         }
     }
 
+    // Successfully re-executing the statement must reset the "end of rowset"
+    // flag, we might be able to fetch data again now.
+    endOfRowSet_ = false;
+
     if (sqldap_->sqld)
     {
         // query may return some data

src/backends/firebird/test/test-firebird.cpp

@@ -370,18 +370,24 @@ void test6()
     }
 
     {
-        char c='a', c1, c2;
+        char c, c1, c2;
 
         statement st = (sql.prepare <<
                         "select p1,p2 from test6 order by p1", into(c1), into(c2));
 
-        st.execute();
-        while (st.fetch())
+        // Verify that fetch after re-executing the same statement works.
+        for (int n = 0; n < 2; ++n)
         {
-            assert(c == c1 && c == c2);
-            ++c;
+            st.execute();
+
+            c='a';
+            while (st.fetch())
+            {
+                assert(c == c1 && c == c2);
+                ++c;
+            }
+            assert(c == 'z'+1);
         }
-        assert(c == 'z'+1);
     }
 
     {