[gnutls-devel] GnuTLS | gnutls_global_deinit() doesn't free all memory after loading system trusted CAs (#1070)

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Miroslav Lichvar created an issue: https://gitlab.com/gnutls/gnutls/-/issues/1070



## Description of problem:
According to the documentation, calling `gnutls_global_deinit()` in an environment with `GNUTLS_NO_EXPLICIT_INIT=1` (or program compiled with `GNUTLS_SKIP_GLOBAL_INIT`) is supposed to free all memory used by gnutls. This doesn't seem to happen in my application, which is normally expected to make a few TLS sessions on start and then run for a long time without TLS. Maybe my program is buggy (see the reproducer), or there is a way to force gnutls to deallocate the memory, but I couldn't find it.

## Version of gnutls used:
gnutls-3.6.14

## Distributor of gnutls (e.g., Ubuntu, Fedora, RHEL)
Fedora 32

## How reproducible:
Always

Steps to Reproduce:
 * compile and run the following program in valgrind:
```
#include <gnutls/gnutls.h>
GNUTLS_SKIP_GLOBAL_INIT

int main() {
  gnutls_certificate_credentials_t credentials = NULL;

  gnutls_global_init();
  gnutls_certificate_allocate_credentials(&credentials);
  gnutls_certificate_set_x509_system_trust(credentials);
  gnutls_certificate_free_credentials(credentials);
  gnutls_global_deinit();
  return 0;
}
```

## Actual results:
```
...
==2640878==    by 0x5CDE3F6: ??? (in /usr/lib64/pkcs11/p11-kit-trust.so)
==2640878==    by 0x5CD9CE4: ??? (in /usr/lib64/pkcs11/p11-kit-trust.so)
==2640878==    by 0x490DDD1: ??? (in /usr/lib64/libgnutls.so.30.28.0)
==2640878==    by 0x490FE3E: ??? (in /usr/lib64/libgnutls.so.30.28.0)
==2640878==    by 0x4910BFF: gnutls_pkcs11_obj_list_import_url4 (in /usr/lib64/libgnutls.so.30.28.0)
==2640878==    by 0x4910D59: gnutls_pkcs11_obj_list_import_url3 (in /usr/lib64/libgnutls.so.30.28.0)
==2640878==    by 0x49686C3: gnutls_x509_trust_list_add_trust_file (in /usr/lib64/libgnutls.so.30.28.0)
==2640878==    by 0x48D9E70: gnutls_x509_trust_list_add_system_trust (in /usr/lib64/libgnutls.so.30.28.0)
==2640878==    by 0x4010A5: main (in /home/miros/a.out)
==2640878== 
==2640878== LEAK SUMMARY:
==2640878==    definitely lost: 0 bytes in 0 blocks
==2640878==    indirectly lost: 0 bytes in 0 blocks
==2640878==      possibly lost: 0 bytes in 0 blocks
==2640878==    still reachable: 5,408,309 bytes in 44,429 blocks
==2640878==         suppressed: 0 bytes in 0 blocks
==2640878== 
==2640878== For lists of detected and suppressed errors, rerun with: -s
==2640878== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)
```

## Expected results:

All memory freed.

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