Verify memory address range validity in GDBRemoteCommunicationClient

Summary:
This aims to verify the validity of the response from the debugging
server in GDBRemoteCommunicationClient::GetMemoryRegionInfo. I was
working with ds2 (https://github.com/facebook/ds2) and encountered a bug
that caused the server's response to have a 'size' value of 0, which
caused lldb to behave incorrectly.

Reviewers: k8stone, labath, clayborg

Reviewed By: labath, clayborg

Subscribers: clayborg, sas, lldb-commits

Differential Revision: https://reviews.llvm.org/D31485

Change by Alex Langford <apl@fb.com>

llvm-svn: 299239