Add missing nullptr check

Author: adrian-prantl

lldb/source/Plugins/TypeSystem/Swift/SwiftASTContext.cpp

@@ -3271,6 +3271,8 @@ class SwiftDWARFImporterDelegate : public swift::DWARFImporterDelegate {
       for (size_t i = 0; i != images.GetSize(); ++i) {
         auto module_sp = images.GetModuleAtIndex(i);
         auto *swift_ast_ctx = GetModuleSwiftASTContext(*module_sp);
+	if (!swift_ast_ctx)
+          continue;
         auto *dwarf_imp = static_cast<SwiftDWARFImporterDelegate *>(
             swift_ast_ctx->GetDWARFImporterDelegate());
         if (!dwarf_imp || dwarf_imp == this)