[scudo][Fuchsia] Don't assume MapPlatformData::Vmar is valid

After https://reviews.llvm.org/D129237, the assumption
that any non-null data contains a valid vmar handle is no
longer true. Generally this code here needs cleanup, but
in the meantime this fixes errors on Fuchsia.

Differential Revision: https://reviews.llvm.org/D129331