Heap buffer overflow in kilo's editor_update_syntax(…)

Hi,

Kilo appears to have a heap buffer overflow triggered by the `memcmp` call [on line 475 of kilo.c](https://github.com/antirez/kilo/blob/master/kilo.c#L475):

```
int j;
for (j = 0; keywords[j]; j++) {
int klen = strlen(keywords[j]);
int kw2 = keywords[j][klen-1] == '|';
if (kw2) klen--;

if (!memcmp(p,keywords[j],klen) &&
```

The signature of `memcmp`:

```
int memcmp(const void *s1, const void *s2, size_t n);
```

`memcmp` operates under the assumption that `s1` and `s2` are at least `n` bytes long each.

This assumption clearly holds for the second argument (`keywords[j]`), but there are no checks in place that makes sure that this assumption holds also for the first argument (`p`).

The heap overflow can be verified by compiling `kilo` with ASAN enabled:

```
$ git clone https://github.com/antirez/kilo.git
$ cd kilo
$ clang -fsanitize=address -o kilo kilo.c
$ ./kilo kilo.c
ERROR: AddressSanitizer: heap-buffer-overflow on address 0x60300000dcd5 at pc 0x0001029f16f9 bp 0x7fff5d233440 sp 0x7fff5d232c00
READ of size 6 at 0x60300000dcd5 thread T0
#0 0x1029f16f8 in wrap_memcmp (libclang_rt.asan_osx_dynamic.dylib+0x116f8)
#1 0x1029d08db in editorUpdateSyntax (kilo+0x1000048db)
#2 0x1029d18fb in editorUpdateRow (kilo+0x1000058fb)
#3 0x1029d1d86 in editorInsertRow (kilo+0x100005d86)
#4 0x1029d3c85 in editorOpen (kilo+0x100007c85)
#5 0x1029d6ae7 in main (kilo+0x10000aae7)
#6 0x7fff8956c5ac in start (libdyld.dylib+0x35ac)
#7 0x1 ()

0x60300000dcd5 is located 0 bytes to the right of 21-byte region [0x60300000dcc0,0x60300000dcd5)
allocated by thread T0 here:
#0 0x102a289c0 in wrap_malloc (libclang_rt.asan_osx_dynamic.dylib+0x489c0)
#1 0x1029d1303 in editorUpdateRow (kilo+0x100005303)
#2 0x1029d1d86 in editorInsertRow (kilo+0x100005d86)
#3 0x1029d3c85 in editorOpen (kilo+0x100007c85)
#4 0x1029d6ae7 in main (kilo+0x10000aae7)
#5 0x7fff8956c5ac in start (libdyld.dylib+0x35ac)
#6 0x1 ()
```