Fix heap-buffer-overflow in keyword highlighting (#36)

[Nicolas0315]

Summary

Fix a heap-buffer-overflow in editorUpdateSyntax() triggered by memcmp reading past the end of the rendered line buffer during keyword matching.

Root Cause

The keyword matching loop uses memcmp(p, keywords[j], klen) without checking that at least klen bytes remain from the current position p to the end of the line. When a short line ends with a prefix that partially matches a keyword (e.g. a line containing just int being compared against a 6-byte keyword), memcmp reads past the allocated buffer.

Fix

Add a bounds check before the memcmp call:

if (klen <= row->rsize-i &&
    !memcmp(p,keywords[j],klen) &&
    is_separator(*(p+klen)))

This ensures we only compare within the allocated line buffer. Short-circuit evaluation means memcmp is never called when there aren't enough bytes remaining.

Verification

Compiled with AddressSanitizer:

clang -fsanitize=address -O1 -g -o kilo kilo.c
./kilo kilo.c  # No ASAN errors

Fixes #36