glibc: fix Segmentation fault in gethostid.c

glibc: fix Segmentation fault in gethostid.c

Linux gethostid: Check for NULL value from gethostbyname_r [BZ #23679]

A NULL value can happen with certain gethostbyname_r failures.

Before this patch, there is a Segmentation fault
as below:
 # /mybuild/hostid
 Segmentation fault
 # gdb /mybuild/hostid
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 Reading symbols from /mybuild/hostid...done.
 (gdb) r
 Starting program: /mybuild/hostid

 Program received signal SIGSEGV, Segmentation fault.
 0x00007ffff7f0330e in gethostid () at ../sysdeps/unix/sysv/linux/gethostid.c:125
 125	  memcpy (&in, hp->h_addr,
 (gdb) bt
 #0  0x00007ffff7f0330e in gethostid () at ../sysdeps/unix/sysv/linux/gethostid.c:125
 #1  0x0000555555555159 in main ()

 # cat /mybuild/hostid.c
 #include <stdio.h>
 #include <unistd.h>

 int
 main(int argc, char *argv[])
 {
    long hostid;
    hostid = gethostid();
    printf("the hostid is %ld\n", hostid);

 }

Signed-off-by: Mingli Yu <Mingli.Yu@windriver.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>