grp/grp_merge.c alignment fix

[DJ Delorie <dj at redhat dot com>]

Fixes https://sourceware.org/bugzilla/show_bug.cgi?id=21654

I have a test for this in my nss_tests patch (posted separately),
which is a very large patch set (the individual test requires the new
framework) and thus I chose to keep it as a separate patch in case
this patch was accepted during the freeze and the testuite change was
deferred.

Tested on x86_64, where it fails the test I wrote before, and not
after, patching.

Note: I did not try to rewrite grp_merge.c using Florian's new
alloc_buffer code, as I felt that was inappropriate for this stage of
development (post-freeze).

	[BZ #21654]
	* grp/grp_merge.c (__copy_grp): Make sure pointers-to-not-char
	are properly aligned.
	(__merge_grp): Likewise.

diff --git a/grp/grp-merge.c b/grp/grp-merge.c
index 77c494d..d6a53cd 100644
--- a/grp/grp-merge.c
+++ b/grp/grp-merge.c
@@ -85,6 +85,14 @@ __copy_grp (const struct group srcgrp, const size_t buflen,
     }
   members[i] = NULL;
 
+  /* Align for pointers.  We can't simply align C because we need to
+     align destbuf[c].  */
+  if (((uintptr_t)destbuf + c) & (__alignof__(char **) - 1))
+    {
+      uintptr_t mis_align = ((uintptr_t)destbuf + c) & (__alignof__(char **) - 1);
+      c += __alignof__(char **) - mis_align;
+    }
+
   /* Copy the pointers from the members array into the buffer and assign them
      to the gr_mem member of destgrp.  */
   destgrp->gr_mem = (char **) &destbuf[c];
@@ -168,6 +176,14 @@ __merge_grp (struct group *savedgrp, char *savedbuf, char *savedend,
   /* Add the NULL-terminator.  */
   members[savedmemcount + memcount] = NULL;
 
+  /* Align for pointers.  We can't simply align C because we need to
+     align savedbuf[c].  */
+  if (((uintptr_t)savedbuf + c) & (__alignof__(char **) - 1))
+    {
+      uintptr_t mis_align = ((uintptr_t)savedbuf + c) & (__alignof__(char **) - 1);
+      c += __alignof__(char **) - mis_align;
+    }
+
   /* Copy the member array back into the buffer after the member list and free
      the member array.  */
   savedgrp->gr_mem = (char **) &savedbuf[c];