According to LibUnwind:src/x86_64/Gstep.c:85 (HEAD=c91974f30feac05055621e33ca101a371236c786), the x86_64 ABI states that the end of the call stack is determined by either a null RBP or an undefined return-address column in the DWARF.
It now actually returns the context at the call site of the function, instead of somewhere inside DwarfInterpret