Fix tuple getter in std unique pointer pretty-printer

Summary: Check case when _M_t child member is not present.

Reviewers: labath, tberghammer

Reviewed By: labath, tberghammer

Differential Revision: https://reviews.llvm.org/D47932
Patch by Aleksandr Urakov <aleksandr.urakov@jetbrains.com>.

llvm-svn: 334411
diff --git a/lldb/source/Plugins/Language/CPlusPlus/LibStdcppUniquePointer.cpp b/lldb/source/Plugins/Language/CPlusPlus/LibStdcppUniquePointer.cpp
index dde7597..9d46e3e 100644
--- a/lldb/source/Plugins/Language/CPlusPlus/LibStdcppUniquePointer.cpp
+++ b/lldb/source/Plugins/Language/CPlusPlus/LibStdcppUniquePointer.cpp
@@ -67,6 +67,8 @@
 
   ValueObjectSP obj_child_sp =
       valobj_sp->GetChildMemberWithName(ConstString("_M_t"), true);
+  if (!obj_child_sp)
+      return nullptr;
 
   ValueObjectSP obj_subchild_sp =
       obj_child_sp->GetChildMemberWithName(ConstString("_M_t"), true);