i#3182: check for NULL sigaction in handle_sigreturn (#3188)
i#3182: check for NULL sigaction in handle_sigreturn (#3188)

A NULL app_sigaction was observed once in handle_sigreturn.  It is not
clear how it happened: otherwise I would add a test.  For now we add a
defensive check.

Fixes #3182
AmesianX/dynamorio,AmesianX/dynamorio,AmesianX/dynamorio,AmesianX/dynamorio,AmesianX/dynamorio