Do not delete system trace session with missing parent process (fix for #8180) (#8179)

* Do not compact system session with missing parent process

* Replace assert with fb_assert

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Co-authored-by: Artyom Abakumov <artyom.abakumov@red-soft.ru>

Author: 2 people

src/jrd/trace/TraceConfigStorage.cpp

@@ -163,6 +163,7 @@ void ConfigStorage::shutdown()
 
 	{
 		StorageGuard guard(this);
+		fb_assert(m_sharedMemory->getHeader()->cnt_uses != 0);
 		--(m_sharedMemory->getHeader()->cnt_uses);
 		if (m_sharedMemory->getHeader()->cnt_uses == 0)
 		{
@@ -473,8 +474,10 @@ void ConfigStorage::compact()
 	for (TraceCSHeader::Slot* slot = header->slots; slot < header->slots + header->slots_cnt; slot++)
 	{
 		if (slot->used && slot->ses_pid != pid &&
+			((slot->ses_flags & trs_system) == 0) && // System sessions are shared for multiple connections so they may live without the original process
 			!ISC_check_process_existence(slot->ses_pid))
 		{
+			fb_assert(header->cnt_uses != 0);
 			header->cnt_uses--; // Process that created trace session disappeared, count it out
 			markDeleted(slot);
 		}