[lldb/api] Add checks for StackFrame::GetRegisterContext calls (NFC)
This patch fixes a crash that is happening because of a null pointer
dereference in SBFrame.
StackFrame::GetRegisterContext says explicitly that you might not get
a valid RegisterContext back but the pointer wasn't tested before,
resulting in crashes. This should solve the issue.
Differential Revision: https://reviews.llvm.org/D83343
Signed-off-by: Med Ismail Bennani <firstname.lastname@example.org>