[clang-format] Don't crash on malformed preprocessor conditions

Previously the program would crash on this input:

```
#else
#if X
```

The problem was that in `parsePPElse`, `PPBranchLevel` would be
incremented when the first `#else` gets parsed, but `PPLevelBranchCount`
and `PPLevelBranchIndex` would not be updated together.

I found the problem when working on D135740.

Differential Revision: https://reviews.llvm.org/D135972