Prevent reading past start of Scintilla buffer

A quick check seemed to show Scintilla does not check the minimum
value is positive, so just check here to be safe and log message
if it fails.

src/editor.c

@@ -665,6 +665,7 @@ static gboolean match_last_chars(ScintillaObject *sci, gint pos, const gchar *st
 	gchar *buf;
 
 	g_return_val_if_fail(len < 100, FALSE);
+	g_return_val_if_fail(len <= pos, FALSE);
 
 	buf = g_alloca(len + 1);
 	sci_get_text_range(sci, pos - len, pos, buf);