llvm-undname: Fix a crash-on-invalid

Found by oss-fuzz, fixes issues 12435 and 12438 on oss-fuzz.

Differential Revision: https://reviews.llvm.org/D60202

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@357646 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Demangle/MicrosoftDemangle.cpp

@@ -1276,12 +1276,16 @@ StringLiteralError:
   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);
 

test/Demangle/invalid-manglings.test

@@ -8,4 +8,9 @@
 ?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