llvm-undname: Fix an assert-on-invalid, found by oss-fuzz

llvm-undname: Fix an assert-on-invalid, found by oss-fuzz

If a template parameter refers to a pointer to member, but the mangling
of that was a string literal instead of a real symbol, llvm-undname used
to crash instead of rejecting the input.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@361402 91177308-0d34-0410-b5e6-96231b3b80d8