short circuit out of a few places where we would allocate zero bytes

short circuit out of a few places where we would allocate zero bytes

dietlibc versions of malloc, calloc and realloc all return NULL if
they're told to allocate 0 bytes, causes the x* wrappers to die().

There are several more places where these calls could end up asking
for 0 bytes, too...

Maybe simply not die()-ing in the x* wrappers if 0/NULL is returned
when the requested size is zero is a safer and easier way to go.

Signed-off-by: Eric Wong <normalperson@yhbt.net>
Signed-off-by: Junio C Hamano <junkio@cox.net>