avisynth: Properly destruct the IScriptEnvironment

Starting with AVISYNTH_INTERFACE_VERSION=5, this is how script
environments should be deleted. The previous code was causing crashes
when unloading AviSynth in certain scenarios, such as when failing to
open a file due to an incorrect path.

src/avisynth_wrap.cpp

@@ -105,7 +105,7 @@ AviSynthWrapper::AviSynthWrapper() {
 
 AviSynthWrapper::~AviSynthWrapper() {
 	if (!--avs_refcount) {
-		delete env;
+		env->DeleteScriptEnvironment();
 #ifdef _WIN32
 		FreeLibrary(hLib);
 #else