BAN: Return UTF::invalid from byte_length instead of 0

userspace/libraries/LibC/stdlib.cpp

@@ -560,9 +560,12 @@ int mblen(const char* s, size_t n)
 		case LOCALE_POSIX:
 			return 1;
 		case LOCALE_UTF8:
-			if (const auto bytes = BAN::UTF8::byte_length(*s); n >= bytes)
-				return bytes;
-			return -1;
+			const auto bytes = BAN::UTF8::byte_length(*s);
+			if (bytes == BAN::UTF8::invalid)
+				return -1;
+			if (n < bytes)
+				return -1;
+			return bytes;
 	}
 	ASSERT_NOT_REACHED();
 }

userspace/libraries/LibFont/PSF.cpp

@@ -176,7 +176,7 @@ namespace LibFont
 
 					uint32_t len = BAN::UTF8::byte_length(bytes[0]);
 
-					if (len == 0)
+					if (len == BAN::UTF8::invalid)
 					{
 						invalid_utf = true;
 						byte_index = 0;

userspace/programs/Terminal/Terminal.cpp

@@ -748,7 +748,7 @@ Rectangle Terminal::putchar(uint8_t ch)
 	m_utf8_bytes[m_utf8_index++] = ch;
 
 	const size_t utf8_len = BAN::UTF8::byte_length(m_utf8_bytes[0]);
-	if (utf8_len == 0)
+	if (utf8_len == BAN::UTF8::invalid)
 	{
 		dwarnln("invalid utf8 leading byte 0x{2H}", ch);
 		m_utf8_index = 0;