account for errors importing ObjC method as async prop

forgot to check for errors when importing an
ObjC method as an async computed property.

rdar://109377788
(cherry picked from commit f3c4996)

Author: kavon

lib/ClangImporter/ImportType.cpp

@@ -2864,13 +2864,17 @@ ImportedType ClangImporter::Implementation::importEffectfulPropertyType(
 
   // Import the parameter list and result type.
   ParameterList *bodyParams = nullptr;
-  ImportedType importedType;
-
   auto methodReturnType = importMethodParamsAndReturnType(
       dc, decl, decl->parameters(), false,
       isFromSystemModule, &bodyParams, name,
       asyncConvention, errorConvention, kind);
 
+  // was there a problem during import?
+  if (!methodReturnType)
+    return ImportedType();
+
+  assert(bodyParams);
+
   // getter mustn't have any parameters!
   if (bodyParams->size() != 0) {
     return ImportedType();