llvm-undname: Fix a crash-on-invalid

[thakis]

Summary

Found by oss-fuzz, fixes issues 12435 and 12438 on oss-fuzz.
Also at least helps with issue 12897.

llvm/lib/Demangle/MicrosoftDemangle.cpp

   std::free(ResultBuffer);
   return Result;
 
 StringLiteralError:
   Error = true;
   return nullptr;
 }
 
+// Returns MangledName's prefix before the first '@', or an error if
+// MangledName contains no '@' or the prefix has length 0.
 StringView Demangler::demangleSimpleString(StringView &MangledName,
                                            bool Memorize) {
   StringView S;
   for (size_t i = 0; i < MangledName.size(); ++i) {
     if (MangledName[i] != '@')
       continue;
+    if (i == 0)
+      break;
     S = MangledName.substr(0, i);
     MangledName = MangledName.dropFront(i + 1);
 
     if (Memorize)
       memorizeString(S);
     return S;
   }
 

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
+; CHECK-NEXT: error: Invalid mangled name
+
+?@@8
+; CHECK-EMPTY:
+; CHECK-NEXT: ?@@8
+; CHECK-NEXT: error: Invalid mangled name