swift-demangle: don't crash on an old pre-swift-2 mangled name

rdar://problem/27248817

Author: eeckstein

lib/Basic/Demangle.cpp

@@ -1494,6 +1494,8 @@ class Demangler {
 
     if (Mangled.nextIf('S')) {
       assocTy = demangleSubstitutionIndex();
+      if (!assocTy)
+        return nullptr;
       if (assocTy->getKind() != Node::Kind::DependentAssociatedTypeRef)
         return nullptr;
     } else {

test/Demangle/Inputs/manglings.txt

@@ -222,4 +222,5 @@ _TFC4testP33_83378C430F65473055F1BD53F3ADCDB71C5doFoofT_T_ ---> test.(C in _8337
 _TFVV15nested_generics5Lunch6DinnerCfT11firstCoursex12secondCourseGSqqd___9leftoversx14transformationFxqd___GS1_x_qd___ ---> nested_generics.Lunch.Dinner.init (firstCourse : A, secondCourse : A1?, leftovers : A, transformation : (A) -> A1) -> nested_generics.Lunch<A>.Dinner<A1>
 _TFVFC15nested_generics7HotDogs11applyRelishFT_T_L_6RelishCfT8materialx_GS1_x_ ---> nested_generics.HotDogs.(applyRelish () -> ()).(Relish #1).init (material : A) -> nested_generics.HotDogs.(applyRelish () -> ()).(Relish #1)<A>
 _TFVFE15nested_genericsSS3fooFT_T_L_6CheeseCfT8materialx_GS0_x_ ---> (extension in nested_generics):Swift.String.(foo () -> ()).(Cheese #1).init (material : A) -> (extension in nested_generics):Swift.String.(foo () -> ()).(Cheese #1)<A>
+_TTWOE5imojiCSo5Imoji14ImojiMatchRankS_9RankValueS_FS2_g9rankValueqq_Ss16RawRepresentable8RawValue ---> _TTWOE5imojiCSo5Imoji14ImojiMatchRankS_9RankValueS_FS2_g9rankValueqq_Ss16RawRepresentable8RawValue