tweaks: reduce the maximum character length from six bytes to four

In UTF-8 valid multibyte characters are at most four bytes long,
and now that we no longer make use of mblen() and mbtowc() from
the underlying system, we won't get five- or six-byte sequences
mistakenly reported as valid (by glibc).  So it is always enough
to reserve space for just four bytes per character.

src/definitions.h

@@ -111,9 +111,9 @@
 #define REPLACING  1
 #define INREGION   2
 
-/* In UTF-8 a character is at most six bytes long. */
 #ifdef ENABLE_UTF8
-#define MAXCHARLEN  6
+/* In UTF-8 a valid character is at most four bytes long. */
+#define MAXCHARLEN  4
 #else
 #define MAXCHARLEN  1
 #endif

src/nano.c

@@ -1804,12 +1804,6 @@ int main(int argc, char **argv)
 	textdomain(PACKAGE);
 #endif
 
-#if defined(ENABLE_UTF8) && !defined(NANO_TINY)
-	if (MB_CUR_MAX > MAXCHARLEN)
-		fprintf(stderr, "Unexpected large character size: %i bytes"
-						" -- please report a bug\n", (int)MB_CUR_MAX);
-#endif
-
 	/* Set sensible defaults, different from what Pico does. */
 	SET(NO_WRAP);
 	SET(SMOOTH_SCROLL);