[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