[lldb] Add missing nullptr checks to AppleObjCRuntime::GetBacktraceThreadFromException

Summary:
We got a few crash reports where LLDB crashes while derefencing the `frames_value` shared_ptr in the AppleObjCRuntime::GetBacktraceThreadFromException. `GetChildMemberWithName` returns a nullptr when an error occurs, so this seems to be just a missing nullptr check.

This patch adds that nullptr check and the other ones in the similar code directly below.

Fixes rdar://62174039

Reviewers: jingham, kubamracek

Reviewed By: jingham

Subscribers: abidh, JDevlieghere

Differential Revision: https://reviews.llvm.org/D78798

lldb/source/Plugins/LanguageRuntime/ObjC/AppleObjCRuntime/AppleObjCRuntime.cpp

@@ -501,19 +501,32 @@ ValueObjectSP AppleObjCRuntime::GetExceptionObjectForThread(
   return ValueObjectSP();
 }
 
+/// Utility method for error handling in GetBacktraceThreadFromException.
+/// \param msg The message to add to the log.
+/// \return An invalid ThreadSP to be returned from
+///         GetBacktraceThreadFromException.
+LLVM_NODISCARD
+static ThreadSP FailExceptionParsing(llvm::StringRef msg) {
+  Log *log(GetLogIfAllCategoriesSet(LIBLLDB_LOG_LANGUAGE));
+  LLDB_LOG(log, "Failed getting backtrace from exception: {0}", msg);
+  return ThreadSP();
+}
+
 ThreadSP AppleObjCRuntime::GetBacktraceThreadFromException(
     lldb::ValueObjectSP exception_sp) {
   ValueObjectSP reserved_dict =
       exception_sp->GetChildMemberWithName(ConstString("reserved"), true);
-  if (!reserved_dict) return ThreadSP();
+  if (!reserved_dict)
+    return FailExceptionParsing("Failed to get 'reserved' member.");
 
   reserved_dict = reserved_dict->GetSyntheticValue();
-  if (!reserved_dict) return ThreadSP();
+  if (!reserved_dict)
+    return FailExceptionParsing("Failed to get synthetic value.");
 
   TypeSystemClang *clang_ast_context =
       TypeSystemClang::GetScratch(*exception_sp->GetTargetSP());
   if (!clang_ast_context)
-    return ThreadSP();
+    return FailExceptionParsing("Failed to get scratch AST.");
   CompilerType objc_id =
       clang_ast_context->GetBasicType(lldb::eBasicTypeObjCID);
   ValueObjectSP return_addresses;
@@ -554,15 +567,22 @@ ThreadSP AppleObjCRuntime::GetBacktraceThreadFromException(
     }
   }
 
-  if (!return_addresses) return ThreadSP();
+  if (!return_addresses)
+    return FailExceptionParsing("Failed to get return addresses.");
   auto frames_value =
       return_addresses->GetChildMemberWithName(ConstString("_frames"), true);
+  if (!frames_value)
+    return FailExceptionParsing("Failed to get frames_value.");
   addr_t frames_addr = frames_value->GetValueAsUnsigned(0);
   auto count_value =
       return_addresses->GetChildMemberWithName(ConstString("_cnt"), true);
+  if (!count_value)
+    return FailExceptionParsing("Failed to get count_value.");
   size_t count = count_value->GetValueAsUnsigned(0);
   auto ignore_value =
       return_addresses->GetChildMemberWithName(ConstString("_ignore"), true);
+  if (!ignore_value)
+    return FailExceptionParsing("Failed to get ignore_value.");
   size_t ignore = ignore_value->GetValueAsUnsigned(0);
 
   size_t ptr_size = m_process->GetAddressByteSize();
@@ -574,7 +594,8 @@ ThreadSP AppleObjCRuntime::GetBacktraceThreadFromException(
     pcs.push_back(pc);
   }
 
-  if (pcs.empty()) return ThreadSP();
+  if (pcs.empty())
+    return FailExceptionParsing("Failed to get PC list.");
 
   ThreadSP new_thread_sp(new HistoryThread(*m_process, 0, pcs));
   m_process->GetExtendedThreadList().AddThread(new_thread_sp);