[Clang] [NFC] Prevent null pointer dereference in Sema::InstantiateFunctionDefinition (#89801)

In the lambda function within
clang::Sema::InstantiateFunctionDefinition, the return value of a
function that may return null is now checked before dereferencing to
avoid potential null pointer dereference issues which can lead to
crashes or undefined behavior in the program.
GitOrigin-RevId: e58dcf135f3a6e453e7b123642675e39b8527f2d
1 file changed