llvm-undname: Fix another crash-on-invalid found by oss-fuzz

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@358363 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Demangle/MicrosoftDemangle.cpp

@@ -654,7 +654,10 @@ Demangler::translateIntrinsicFunctionCode(char CH,
 IdentifierNode *
 Demangler::demangleFunctionIdentifierCode(StringView &MangledName,
                                           FunctionIdentifierCodeGroup Group) {
-  assert(!MangledName.empty());
+  if (MangledName.empty()) {
+    Error = true;
+    return nullptr;
+  }
   switch (Group) {
   case FunctionIdentifierCodeGroup::Basic:
     switch (char CH = MangledName.popFront()) {