test-skeleton.c (xrealloc): Support realloc-as-free

If the requested size is zero, realloc returns NULL, but the
deallocation is still successful, unless the pointer is also
NULL, when realloc behaves as malloc (0).

ChangeLog

@@ -1,3 +1,7 @@
+2016-06-23  Florian Weimer  <fweimer@redhat.com>
+
+	* test-skeleton.c (xrealloc): Support deallocation with n == 0.
+
 2016-06-23  Florian Weimer  <fweimer@redhat.com>
 
 	* test-skeleton.c (xmalloc, xcalloc, xrealloc): Mark as

test-skeleton.c

@@ -109,10 +109,10 @@ __attribute__ ((unused))
 static void *
 xrealloc (void *p, size_t n)
 {
-  p = realloc (p, n);
-  if (p == NULL)
+  void *result = realloc (p, n);
+  if (result == NULL && (n > 0 || p == NULL))
     oom_error ("realloc", n);
-  return p;
+  return result;
 }
 
 /* Write a message to standard output.  Can be used in signal