Fix demangling crasher.  The crasher involved nested <encoding> involving parameter packs, which exposed a logic bug causing an empty vector<string> to be accessed with back().  In addition to fixing the bug, I've inserted numerous preemptive checks for similar bugs in the hopes that if another bug is uncovered, the bug results in an invalid mangled string instead of a demangler crash.  Test suite updated with string that was causing the crash.

git-svn-id: https://llvm.org/svn/llvm-project/libcxxabi/trunk@197063 91177308-0d34-0410-b5e6-96231b3b80d8