Syntax Highlighting Segfault

# How to reproduce
1. Create any `*.c` file with 4 empty lines.
2. Open the file with `kilo`, go to the 3rd line, press delete twice
3. Now 2 lines remain, and cursor is at the first line. Move to next line
4. Enter any character, e.g., `a`
5. Segfault

#81 resolves this, please consider merging it.