runtime: don't crash if signal handler info argument is nil

runtime: don't crash if signal handler info argument is nil

    Apparently on Solaris 10 a SA_SIGINFO signal handler can be invoked with
    a nil info argument.  I would not have believed it but I've now seen it
    happen, and the sigaction man page actually says "If the second argument
    is not equal to NULL, it points to a siginfo_t structure...."  So, if
    that happens, don't crash.

    Also fix another case where we want to make sure that &T{} does not
    allocate.

    Reviewed-on: https://go-review.googlesource.com/33150

From-SVN: r242403