The stack might produce data (such as as the QML context in
native mixed debugging) that is needed for locals display.
While this doesn't work yet with LLDB anyway, the change
suppresses one soft access when accessing the top frame of
an empty stack (the usual case on the first update).

Change-Id: If931fbe940b8cda01cc6cc34429a2fcb33cd8d19
Reviewed-by: Christian Stenger <christian.stenger@theqtcompany.com>