[LSan] Fix one source of stale segments in the process memory mapping.

Summary:
Load process memory map after updating the same cache to reflect the
umap happening in the process of updating.
Also clear out the buffer in case of failed read of /proc/self/maps (not
the source of stale segments, but can lead to the similar crash).

Reviewers: eugenis

Subscribers: llvm-commits, kubamracek

Differential Revision: https://reviews.llvm.org/D40529

llvm-svn: 319237