Virtuoso 7.2.12 crashes with the `XPF_EXTENSION` function.

Environment: Ubuntu 20.04, docker image `openlink/virtuoso-opensource-7:7.2.12`.

PoC:
```sql
SELECT XPF_EXTENSION('', 1, 1);
```

Backtrace:
```
#0 0x7f876381c915 (__nss_database_lookup+0x25295)
#1 0x59d4b6 (sch_split_name_len+0x56)
#2 0x59d7da (sch_split_name+0x3a)
#3 0x60accc (sch_name_to_object+0x7c)
#4 0xb8f9d6 (bif_xpf_extension+0x106)
#5 0x755af4 (ins_call_bif+0xc4)
#6 0x75d882 (code_vec_run_v+0x25f2)
#7 0x7b9c8f (qn_input+0x38f)
#8 0x7ba136 (qn_send_output+0x236)
#9 0x82c91d (set_ctr_vec_input+0x99d)
#10 0x7b9cce (qn_input+0x3ce)
#11 0x7cb4bb (qr_exec+0x11db)
#12 0x7d8e56 (sf_sql_execute+0x11a6)
#13 0x7d995e (sf_sql_execute_w+0x17e)
#14 0x7e261d (sf_sql_execute_wrapper+0x3d)
#15 0xe29cec (future_wrapper+0x3fc)
#16 0xe315ee (_thread_boot+0x11e)
#17 0x7f87639e3609 (start_thread+0xd9)
#18 0x7f87637b3353 (clone+0x43)
```
Virtuoso 7.2.12 crashes with the `XPF_EXTENSION` function
This is a GitHub Issue
repo:virtuoso-opensource
owner:openlink
Title : Virtuoso 7.2.12 crashes with the `XPF_EXTENSION` function
Issue date:

--- start body ---
Virtuoso 7.2.12 crashes with the `XPF_EXTENSION` function.

Environment: Ubuntu 20.04, docker image `openlink/virtuoso-opensource-7:7.2.12`.

PoC:
```sql
SELECT XPF_EXTENSION('', 1, 1);
```

Backtrace:
```
#0 0x7f876381c915 (__nss_database_lookup+0x25295)
#1 0x59d4b6 (sch_split_name_len+0x56)
#2 0x59d7da (sch_split_name+0x3a)
#3 0x60accc (sch_name_to_object+0x7c)
#4 0xb8f9d6 (bif_xpf_extension+0x106)
#5 0x755af4 (ins_call_bif+0xc4)
#6 0x75d882 (code_vec_run_v+0x25f2)
#7 0x7b9c8f (qn_input+0x38f)
#8 0x7ba136 (qn_send_output+0x236)
#9 0x82c91d (set_ctr_vec_input+0x99d)
#10 0x7b9cce (qn_input+0x3ce)
#11 0x7cb4bb (qr_exec+0x11db)
#12 0x7d8e56 (sf_sql_execute+0x11a6)
#13 0x7d995e (sf_sql_execute_w+0x17e)
#14 0x7e261d (sf_sql_execute_wrapper+0x3d)
#15 0xe29cec (future_wrapper+0x3fc)
#16 0xe315ee (_thread_boot+0x11e)
#17 0x7f87639e3609 (start_thread+0xd9)
#18 0x7f87637b3353 (clone+0x43)
```
--- end body ---
