Fuzzer: Virtuoso 7.2.12 crashed at `sqlo_union_scope`.
It can also be reproduced with Version 7.2.13-dev.3239-pthreads as of Mar 17 2024 (da40b02).

PoC:
```SQL
SELECT CASE WHEN x THEN ( SELECT CASE WHEN x THEN CONCAT ( ) END x FROM ( SELECT x ) x UNION SELECT 4 x , 5 x ) END FROM ( SELECT 0 x ) x;
```

backtrace:
```c
#0 0x777231 (sqlo_union_scope+0x1f1)
#1 0x77846b (sqlo_scope+0x119b)
#2 0x78bc3e (sqlo_scalar_subq_scope+0xfe)
#3 0x77757a (sqlo_scope+0x2aa)
#4 0x7779c7 (sqlo_scope+0x6f7)
#5 0x78837f (sqlo_select_scope+0xa4f)
#6 0x777783 (sqlo_scope+0x4b3)
#7 0x714661 (sqlo_top_2+0x41)
#8 0x7142b5 (sqlo_top_1+0x135)
#9 0x716066 (sqlo_top_select+0x156)
#10 0x6bf21b (sql_stmt_comp+0x8bb)
#11 0x6c20b2 (sql_compile_1+0x1a62)
#12 0x7d5b40 (stmt_set_query+0x340)
#13 0x7d7a32 (sf_sql_execute+0x922)
#14 0x7d8dbe (sf_sql_execute_w+0x17e)
#15 0x7e1a7d (sf_sql_execute_wrapper+0x3d)
#16 0xe2a11c (future_wrapper+0x3fc)
#17 0xe31a1e (_thread_boot+0x11e)
#18 0x7f3ffb497609 (start_thread+0xd9)
#19 0x7f3ffb267353 (clone+0x43)
```

ways to reproduce (write poc to the file /tmp/test.sql first):
```bash
# remove the old one
docker container rm virtdb_test -f
# start virtuoso through docker
docker run --name virtdb_test -itd --env DBA_PASSWORD=dba  pkleef/virtuoso-opensource-7
# wait the server starting
while ! echo "SELECT 1;" | docker exec -i virtdb_test-dev isql 1111 dba > /dev/null 2>&1; do
    echo "Waiting the server starting..."
    sleep 1
done
# run the poc
cat /tmp/test.sql | docker exec -i virtdb_test isql 1111 dba
```
Fuzzer: Virtuoso 7.2.12 crashed at `sqlo_union_scope`
This is a GitHub Issue
repo:virtuoso-opensource
owner:openlink
Title : Fuzzer: Virtuoso 7.2.12 crashed at `sqlo_union_scope`
Issue date:

--- start body ---
Fuzzer: Virtuoso 7.2.12 crashed at `sqlo_union_scope`.
It can also be reproduced with Version 7.2.13-dev.3239-pthreads as of Mar 17 2024 (da40b02).

PoC:
```SQL
SELECT CASE WHEN x THEN ( SELECT CASE WHEN x THEN CONCAT ( ) END x FROM ( SELECT x ) x UNION SELECT 4 x , 5 x ) END FROM ( SELECT 0 x ) x;
```

backtrace:
```c
#0 0x777231 (sqlo_union_scope+0x1f1)
#1 0x77846b (sqlo_scope+0x119b)
#2 0x78bc3e (sqlo_scalar_subq_scope+0xfe)
#3 0x77757a (sqlo_scope+0x2aa)
#4 0x7779c7 (sqlo_scope+0x6f7)
#5 0x78837f (sqlo_select_scope+0xa4f)
#6 0x777783 (sqlo_scope+0x4b3)
#7 0x714661 (sqlo_top_2+0x41)
#8 0x7142b5 (sqlo_top_1+0x135)
#9 0x716066 (sqlo_top_select+0x156)
#10 0x6bf21b (sql_stmt_comp+0x8bb)
#11 0x6c20b2 (sql_compile_1+0x1a62)
#12 0x7d5b40 (stmt_set_query+0x340)
#13 0x7d7a32 (sf_sql_execute+0x922)
#14 0x7d8dbe (sf_sql_execute_w+0x17e)
#15 0x7e1a7d (sf_sql_execute_wrapper+0x3d)
#16 0xe2a11c (future_wrapper+0x3fc)
#17 0xe31a1e (_thread_boot+0x11e)
#18 0x7f3ffb497609 (start_thread+0xd9)
#19 0x7f3ffb267353 (clone+0x43)
```

ways to reproduce (write poc to the file /tmp/test.sql first):
```bash
# remove the old one
docker container rm virtdb_test -f
# start virtuoso through docker
docker run --name virtdb_test -itd --env DBA_PASSWORD=dba  pkleef/virtuoso-opensource-7
# wait the server starting
while ! echo "SELECT 1;" | docker exec -i virtdb_test-dev isql 1111 dba > /dev/null 2>&1; do
    echo "Waiting the server starting..."
    sleep 1
done
# run the poc
cat /tmp/test.sql | docker exec -i virtdb_test isql 1111 dba
```
--- end body ---
