llvm-undname: Fix stack overflow on almost-valid

If a unsigned with all 4 bytes non-0 was passed to outputHex(), there
were two off-by-ones in it:

- Both MaxPos and Pos left space for the final \0, which left the buffer
  one byte to small. Set MaxPos to 16 instead of 15 to fix.

- The `assert(Pos >= 0);` was after a `Pos--`, move it up one line.

Since valid Unicode codepoints are <= 0x10ffff, this could never really
happen in practice.

Found by oss-fuzz.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@358856 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Demangle/MicrosoftDemangle.cpp

@@ -1071,17 +1071,17 @@ static void outputHex(OutputStream &OS, unsigned C) {
   char TempBuffer[17];
 
   ::memset(TempBuffer, 0, sizeof(TempBuffer));
-  constexpr int MaxPos = 15;
+  constexpr int MaxPos = sizeof(TempBuffer) - 1;
 
-  int Pos = MaxPos - 1;
+  int Pos = MaxPos - 1; // TempBuffer[MaxPos] is the terminating \0.
   while (C != 0) {
     for (int I = 0; I < 2; ++I) {
       writeHexDigit(&TempBuffer[Pos--], C % 16);
       C /= 16;
     }
     TempBuffer[Pos--] = 'x';
-    TempBuffer[Pos--] = '\\';
     assert(Pos >= 0);
+    TempBuffer[Pos--] = '\\';
   }
   OS << StringView(&TempBuffer[Pos + 1]);
 }