Test flags before using "info"

Our debugger signal catcher expects to receive three args, but if
somebody cleared SA_SIGINFO we only get one, and bad things happen
when we try to use the second one.  Test to see if SA_SIGINFO is
still set before we try to use the argument.

Bug: 7272866
Change-Id: If682a2262fe8a575f3802fb0c60a53eea3082456