free thread ctx on uc_vm_free when no longer being ref

[evertonen]

fix #385

[evertonen]

No longer leaking as shown in the description of the issue:

root@33db7cc5518d:/ucode/examples# gcc -g -Wall execute-file.c -lucode
root@33db7cc5518d:/ucode/examples# 
root@33db7cc5518d:/ucode/examples# echo '{% z=x+y; print(z, "\n"); %}' > source.uc
root@33db7cc5518d:/ucode/examples# 
root@33db7cc5518d:/ucode/examples# ./a.out ./source.uc 
579
Program finished successfully.
Function return value is null
root@33db7cc5518d:/ucode/examples# 
root@33db7cc5518d:/ucode/examples# valgrind --leak-check=full --show-leak-kinds=all -s ./a.out ./source.uc
==712== Memcheck, a memory error detector
==712== Copyright (C) 2002-2024, and GNU GPL'd, by Julian Seward et al.
==712== Using Valgrind-3.24.0 and LibVEX; rerun with -h for copyright info
==712== Command: ./a.out ./source.uc
==712== 
579
Program finished successfully.
Function return value is null
==712== 
==712== HEAP SUMMARY:
==712==     in use at exit: 0 bytes in 0 blocks
==712==   total heap usage: 210 allocs, 210 frees, 20,853 bytes allocated
==712== 
==712== All heap blocks were freed -- no leaks are possible
==712== 
==712== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)
root@33db7cc5518d:/ucode/examples#