Garbage in debug output

Nobody has claimed this yet.

When there is not enough video memory, often garbage can be seen in debug output, like this:
```
[h264_nvenc @ 0x7f9f50e48f40] InitializeEncoder failed: out of memory (10): R,̒z �H��D��
I��̀��K)4?�??��:��Ȃt��
��>�ٿ�z��{�J��1�
Error opening video encoder
```

It is produced in this function:
```c
static int nvenc_print_error(AVCodecContext *avctx, NVENCSTATUS err,
const char *error_string)
{
const char *desc;
const char *details = "(no details)";
int ret = nvenc_map_error(err, &desc);

#ifdef NVENC_HAVE_GETLASTERRORSTRING
NvencContext *ctx = avctx->priv_data;
NV_ENCODE_API_FUNCTION_LIST *p_nvenc = &ctx->nvenc_dload_funcs.nvenc_funcs;

if (p_nvenc && ctx->nvencoder)
details = p_nvenc->nvEncGetLastErrorString(ctx->nvencoder);
#endif

av_log(avctx, AV_LOG_ERROR, "%s: %s (%d): %s\n", error_string, desc, err, details);

return ret;
}
```

Garbage is the pointer returned by `nvEncGetLastErrorString` function.
It looks like `nvEncGetLastErrorString` is not thread safe.
Potentially this could be vulnerability.
Given that use of this function is guarded by `NVENC_HAVE_GETLASTERRORSTRING` define, I think we should undefine it at configuration stage to disable it usage.

Start at nvenc_print_error and trace how NVENC_HAVE_GETLASTERRORSTRING is set during configuration. Check the configuration path and the debug-output behavior when encoder initialization fails for lack of video memory; done means the unsafe last-error-string path is disabled and no garbage appears in the logged error.

Written by the indexing model from the issue text.