PVS-Studio warning V557 for QT

static const JoiningPair joining_table[5][4] = { .... };

typedef enum {
    JNone,
    JCausing,
    JDual,
    JRight,
    JTransparent
} Joining;

static Joining getNkoJoining(unsigned short uc)
{
    if (uc < 0x7ca)
        return JNone;
    if (uc <= 0x7ea)
        return JDual;
    if (uc <= 0x7f3)
        return JTransparent;
    if (uc <= 0x7f9)
        return JNone;
    if (uc == 0x7fa)
        return JCausing;
    return JNone;
}

static void getNkoProperties(....)
{
  ....
  Joining j = getNkoJoining(chars[0]);
  ArabicShape shape = joining_table[XIsolated][j].form2;
  ....
}

This suspicious code was found in QT project by PVS-Studio static code analyzer.
Warning message is:
V557 Array overrun is possible. The value of 'j' index could reach 4. harfbuzz-arabic.c 516

PVS-Studio is a static analyzer for detecting bugs in the source code of applications written in C, C++, C++11, C++/CX. Site: http://www.viva64.com/en/pvs-studio/