clang-format: Fix access to uninitialized memory.

With incomplete code, we aren't guaranteed to generated changes for
every token. In that case, we need to assume that even the very first
change can continue a preprocessor directive and initialize values
accordingly.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@231066 91177308-0d34-0410-b5e6-96231b3b80d8