[hwasan] Handle missing /proc/self/maps.

Summary:
Don't crash when /proc/self/maps is inaccessible from main thread.
It's not a big deal, really.

Reviewers: vitalybuka, kcc

Subscribers: kubamracek, llvm-commits

Differential Revision: https://reviews.llvm.org/D50574

llvm-svn: 339607