Thanks to Juan Lang for help.

Based on Dmitry Timoshkov idea.

Instead of trying to infer unsynchronized ones.

This (presumably) fixes a regression introduced by commit
c1032e97.

Bionic doesn't provide one, even when vdso has been unmapped by the
preloader.

This means the resulting rectangle will be short, but we don't have much
choice.  Some apps don't cope properly with the one-past-the-end character.
For example, Excel 2007 gets stuck in an infinite loop.