Do not dereference null generic env

Author: AlexDenisov

patches/swift/do-not-dereference-null-env.patch

@@ -0,0 +1,12 @@
+diff --git a/lib/AST/Type.cpp b/lib/AST/Type.cpp
+index ccb64354851..a5f238bb8f2 100644
+--- a/lib/AST/Type.cpp
++++ b/lib/AST/Type.cpp
+@@ -3557,6 +3557,7 @@ bool ArchetypeType::isRoot() const {
+ 
+ Type ArchetypeType::getExistentialType() const {
+   auto *genericEnv = getGenericEnvironment();
++  if (!genericEnv) return {};
+ 
+   // Opened types hold this directly.
+   if (auto *opened = dyn_cast<OpenedArchetypeType>(this)) {