llvm-undname: Fix oss-fuzz-foudn crash-on-invalid with incomplete special table nodes

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@358367 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Demangle/MicrosoftDemangle.cpp

@@ -276,6 +276,10 @@ Demangler::demangleSpecialTableSymbolNode(StringView &MangledName,
   SpecialTableSymbolNode *STSN = Arena.alloc<SpecialTableSymbolNode>();
   STSN->Name = QN;
   bool IsMember = false;
+  if (MangledName.empty()) {
+    Error = true;
+    return nullptr;
+  }
   char Front = MangledName.popFront();
   if (Front != '6' && Front != '7') {
     Error = true;

test/Demangle/invalid-manglings.test

@@ -119,3 +119,13 @@
 ; CHECK-EMPTY:
 ; CHECK-NEXT: ?foo@?$?_
 ; CHECK-NEXT: error: Invalid mangled name
+
+??_R4
+; CHECK-EMPTY:
+; CHECK-NEXT: ??_R4
+; CHECK-NEXT: error: Invalid mangled name
+
+??_R4foo@@
+; CHECK-EMPTY:
+; CHECK-NEXT: ??_R4foo@@
+; CHECK-NEXT: error: Invalid mangled name