Fix handling of pvalloc(0)

[rdevshp]

pvalloc(0) currently returns NULL and sets errno = ENOMEM. This PR fixes handling of pvalloc(0).

[thestinger]

This is an overly complicated way of doing this.

[rdevshp]

This is an overly complicated way of doing this.

I have switched to checking if the size is zero at the beginning of pvalloc instead.

[thestinger]

Have you verified this is what glibc malloc does?

[rdevshp]

Have you verified this is what glibc malloc does?

I have checked that with glibc's pvalloc, the program

#include <errno.h>
#include <malloc.h>
#include <stdio.h>
#include <stdlib.h>

int main(void) {
    errno = 0;
    void *p = pvalloc(0);
    int saved_errno = errno;
    printf("p=%p errno=%d\n", p, saved_errno);
    free(p);
    return 0;
}

prints a non-null p with errno=0.

[thestinger]

@rdevshp Is it page-aligned though?

[rdevshp]

@rdevshp Is it page-aligned though?

Yes, the pointer that I got from glibc's pvalloc is page-aligned.