libc: zero size allocations are handled improperly

[Lukáš Zaoral]

Hi,
whenever a zero size allocation occurs, DiOS' implementation of {m,c,re}aloc sets errno to ENOMEM, which is a correct behaviour (at least according to POSIX.1-2017). However, just returning NULL in such case is not desirable, because Divine would never find following errors:

1.

#include <stdlib.h>

int main(void)
{
    char *ptr = malloc(0);
    if (ptr == NULL)
        return EXIT_SUCCESS;

    (void) *ptr; /* error */
    free(ptr);
}

2.

#include <stdlib.h>

int main(void)
{
    char *ptr = malloc(0);
    if (ptr == NULL)
        return EXIT_SUCCESS;

    /* possible leak */
}

[mornfall]

The invalid dereference variant is fixed in next. I don't think the leak check is very important, but if you feel otherwise, please open a separate ticket.