cql3: Fix value_for when restriction is impossible

Previously, single_column_restrictions::value_for() assumed that a
column's restriction specifies exactly one value for the column.  But
since 37ebe52, multiple equalities on the same column are allowed,
so the restriction could be a conjunction of conflicting
equalities (eg, c=1 AND c=0).  That violates an assert and crashes
Scylla.

This patch fixes value_for() by gracefully handling the
impossible-restriction case.

Fixes scylladb#7772

Signed-off-by: Dejan Mircevski <dejan@scylladb.com>

cql3/restrictions/single_column_restrictions.hh

@@ -108,6 +108,9 @@ public:
             return bytes_opt{};
         } else {
             const auto values = std::get<expr::value_list>(possible_lhs_values(&cdef, it->second->expression, options));
+            if (values.empty()) {
+                return bytes_opt{};
+            }
             assert(values.size() == 1);
             return values.front();
         }

test/boost/secondary_index_test.cc

@@ -76,6 +76,14 @@ SEASTAR_TEST_CASE(test_secondary_index_clustering_key_query) {
                 { utf8_type->decompose(sstring("dcurrorw@techcrunch.com")) },
                 { utf8_type->decompose(sstring("beassebyv@house.gov")) },
             });
+        }).then([&e] {
+            return e.execute_cql("select country from users where country='France' and country='Denmark'"); // #7772
+        }).then([&e] (shared_ptr<cql_transport::messages::result_message> msg) {
+            assert_that(msg).is_rows().is_empty();
+        }).then([&e] {
+            return e.execute_cql("select country from users where country='Denmark' and country='Denmark'");
+        }).then([&e] (shared_ptr<cql_transport::messages::result_message> msg) {
+            assert_that(msg).is_rows().with_rows({{utf8_type->decompose(sstring("Denmark"))}});
         });
     });
 }