llvm-undname: Fix nullptr deref on invalid structor names in template args

Similar to r358421: A StructorIndentifierNode has a Class field which
is read when printing it, but if the StructorIndentifierNode appears in
a template argument then demangleFullyQualifiedSymbolName() which sets
Class isn't called. Since StructorIndentifierNodes are always leaf
names, we can just reject them as well.

Found by oss-fuzz.

llvm-svn: 358491