[lldb] Fix null pointer crash in LibStdcppTupleSyntheticFrontEnd::Update (#175700)
When displaying a backtrace with std::tuple function arguments,
GetChildAtIndex() can return a null ValueObjectSP. The code was
dereferencing this pointer without checking for null, causing a SIGSEGV
crash when LLDB tried to pretty-print function arguments containing
tuples.
This PR fix the crash and add unit test which would crash before this PR
and only pass after this PR.
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Co-authored-by: George Hu <georgehuyubo@gmail.com>
Co-authored-by: Jonas Devlieghere <jonas@devlieghere.com>