llvm-undname: Fix an assert-on-invalid

[thakis]

Summary

Found by oss-fuzz, fixes issues 12428 and 12429 on oss-fuzz.

llvm/lib/Demangle/MicrosoftDemangle.cpp

SymbolNode *Demangler::demangleSpecialIntrinsic(StringView &MangledName) {
   Error = true;
   return nullptr;
 }
 
 IdentifierNode *
 Demangler::demangleFunctionIdentifierCode(StringView &MangledName) {
   assert(MangledName.startsWith('?'));
   MangledName = MangledName.dropFront();
+  if (MangledName.empty()) {
+    Error = true;
+    return nullptr;
+  }
 
   if (MangledName.consumeFront("__"))
     return demangleFunctionIdentifierCode(
         MangledName, FunctionIdentifierCodeGroup::DoubleUnder);
   else if (MangledName.consumeFront("_"))
     return demangleFunctionIdentifierCode(MangledName,
                                           FunctionIdentifierCodeGroup::Under);
   return demangleFunctionIdentifierCode(MangledName,
case FunctionIdentifierCodeGroup::DoubleUnder:
     return DoubleUnder[Index];
   }
   DEMANGLE_UNREACHABLE;
 }
 
 IdentifierNode *
 Demangler::demangleFunctionIdentifierCode(StringView &MangledName,
                                           FunctionIdentifierCodeGroup Group) {
+  assert(!MangledName.empty());
   switch (Group) {
   case FunctionIdentifierCodeGroup::Basic:
     switch (char CH = MangledName.popFront()) {
     case '0':
     case '1':
       return demangleStructorIdentifier(MangledName, CH == '1');
     case 'B':
       return demangleConversionOperatorIdentifier(MangledName);

llvm/test/Demangle/invalid-manglings.test

 ; Run llvm-undname with invalid inputs and make sure it doesn't crash.
 ; RUN: llvm-undname < %s 2>&1 | FileCheck %s
 
 ?ff@@$$J0YAXAU?$AS_@$0A@PEAU?$AS_@$0A@H@__clang@@@__clang@@@Z
 ; CHECK: ?ff@@$$J0YAXAU?$AS_@$0A@PEAU?$AS_@$0A@H@__clang@@@__clang@@@Z
 ; CHECK-NEXT: error: Invalid mangled name
 
 ?f0@@YAXPEU?$AS_@$00$$CAD@__clang@@@Z
 ; CHECK-EMPTY:
 ; CHECK-NEXT: ?f0@@YAXPEU?$AS_@$00$$CAD@__clang@@@Z
 ; CHECK-NEXT: error: Invalid mangled name
 
 ?@@8
 ; CHECK-EMPTY:
 ; CHECK-NEXT: ?@@8
 ; CHECK-NEXT: error: Invalid mangled name
+
+??
+; CHECK-EMPTY:
+; CHECK-NEXT: ??
+; CHECK-NEXT: error: Invalid mangled name