Fix: crashes to desktop from game scripts when companies disappear

When a game script is in company mode, it pretends to be another company. When
that company disappear (bankruptcy/merger), the game script still uses that
company and it keeps calling functions as if it is that company.

For example, ScriptEngine::IsBuildable internally dereferences Company without
checks, causing a null dereference for any ScriptEngine function when called
from a company scope of a company that has disappeared.

Guard against this by extending the ScriptCompanyScope::IsValid check to also
check for the company still being active.

src/script/api/script_companymode.cpp

@@ -29,7 +29,7 @@ ScriptCompanyMode::~ScriptCompanyMode()
 
 /* static */ bool ScriptCompanyMode::IsValid()
 {
-	return ScriptObject::GetCompany() != OWNER_DEITY;
+	return ::Company::IsValidID(ScriptObject::GetCompany());
 }
 
 /* static */ bool ScriptCompanyMode::IsDeity()