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.

llvm-svn: 358856