clang-format: Fix bug with ENAS_DontAlign and empty lines

This fixes a bug in `ENAS_DontAlign` (introduced in D32733) where blank lines had an EscapedNewlineColumn of 0, causing a subtraction to overflow when converted back to unsigned and leading to runaway memory allocation.

Differential Revision: https://reviews.llvm.org/D36019

llvm-svn: 310539