llvm-undname: Fix nullptr deref on invalid conversion operator names in template args
A ConversionOperatorIdentifierNode has a TargetType which is read when
printing it, but if the ConversionOperatorIdentifierNode appears in a
template argument there's nothing that can provide the TargetType.
Normally the COIN is a symbol (leaf) name and takes its TargetType from the
symbol's type, but in a template argument context the COIN can only be
either a non-leaf name piece or a type, and must hence be invalid.
Similar to the COIN check in demangleDeclarator().
Found by oss-fuzz.