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.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().
 	 */