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()

src/script/api/script_companymode.hpp

@@ -50,7 +50,7 @@ public:
 
 	/**
 	 * Check whether a company mode is valid. In other words, are commands
-	 * being executed under some company.
+	 * being executed under some company and does the company still exist?
 	 * @return true When a company mode is valid.
 	 * @post !ScriptCompanyMode::IsDeity().
 	 */

src/script/api/script_object.cpp

@@ -253,7 +253,7 @@ std::tuple<bool, bool, bool> ScriptObject::DoCommandPrep()
 
 	bool networking = _networking && !_generating_world;
 
-	if (ScriptCompanyMode::IsValid() && !::Company::IsValidID(ScriptObject::GetCompany())) {
+	if (!ScriptCompanyMode::IsDeity() && !ScriptCompanyMode::IsValid()) {
 		ScriptObject::SetLastError(ScriptError::ERR_PRECONDITION_INVALID_COMPANY);
 		return { true, estimate_only, networking };
 	}