Fix a confusion between osigcontext and ucontext_t in the previous commit.
Since an osigcontext is smaller, if you check for a valid (much larger sized)
ucontext_t and it fails, we bogusly would reject the osigcontext as per
rev 1.378. Instead, check for osigcontext range validity first, and
ucontext_t later. This unbreaks Netscape.

Pointed to the right commit by: peter