[lldb] Handle malformed qfThreadInfo reply

If the remote gdbserver's qfThreadInfo reply has a trailing comma,
GDBRemoteCommunicationClient::GetCurrentProcessAndThreadIDs will return
an empty vector of thread ids. This will cause lldb to recurse through
three functions trying to get the list of threads, until it blows its
stack and crashes.

A trailing comma is a malformed response, but it shouldn't cause lldb to
crash. This patch will return the tids received before the malformed
response.

Reviewed By: clayborg, labath

Differential Revision: https://reviews.llvm.org/D109937