Removing condition that will never be met after calls xmalloc and xrealloc

Hi,

Looking in the kexec-tools code I found these conditions that
seems will never be met. Not sure if that was intentional for explicitity,
if it was the case, please disconsider this patch.

xmalloc and xrealloc when fails calls die() that calls exit(1).
Checks for if(!memory) after they are called will never be met that
condition, since the process will be exited after an allocation fail.

Signed-off-by: Leonidas S. Barbosa <kirotawa@gmail.com>
Signed-off-by: Simon Horman <horms@verge.net.au>