itoa base function in C

This is a function that converts an integer value to a null-terminated string using the specified base and stores the result in a char array that I must allocate (with malloc). I have decided not to use malloc. Allowed functions: malloc. Loops: while Please review my code.

char *itoa_base(int value, int base)
{
    static  char rep[] = "0123456789abcdef";
    static  char buf[50];
    char    *ptr;
    int     num;

    ptr = &buf[49];
    *ptr = '\0';
    num = value;
    if (value < 0 && base == 10)
        value *= -1;
    if (value == 0)
        *--ptr = rep[value % base];
    while (value != 0)
    {
        *--ptr = rep[value % base];
        value /= base;
    }
    if (num < 0 && base == 10)
        *--ptr = '-';
    return (ptr);

Input checking

• If the caller passes in a base greater than 17, you will read past the end of rep.

• If the caller passes in a base of 0, you will divide by zero.

• If the caller passes in a base of 1, your code will infinite loop.

• If the caller passes in a negative value but the base is not 10, you will read past the front of rep because your modulus will be negative. You should always make value positive before proceeding with your loop.

Insufficient buffer size

On a system with 64-bit ints and a base of 2, you will need a buffer of size 65 instead of 50. (Or 66 if you allow negative values for all bases).
Code simplification

This code:

if (value == 0)
    *--ptr = rep[value % base];
while (value != 0)
{
    *--ptr = rep[value % base];
    value /= base;
}

could be simplified to:

do {
    *--ptr = rep[value % base];
    value /= base;
} while (value != 0);

I'm not sure from your question whether you were allowed to use a do loop or not. Also, your question says you are supposed to return an allocated buffer but you intentionally didn't, so I'm not sure how to review that part of it.

StackExchange Code Review Q#132591, answer score: 4