tar: don't cross struct member boundaries with OLDGNU_MAGIC

* src/create.c (write_gnu_long_link, start_header): Access
header->buffer + offsetof (struct posix_header, magic), instead of
header->header.magic, when reading or writing the OLDGNU_MAGIC
pattern.  The code violates the C standard without this change,
and GCC warns about this if fortify checking is enabled.  It's not
a bug on traditional (i.e., non-debugging) platforms, but it does
violate the C standard so it should be fixed.  Problem originally
reported by John Emil Karlson in
<http://lists.gnu.org/archive/html/bug-tar/2010-04/msg00023.html>.
* src/list.c (decode_header): Likewise.

src/create.c

@@ -562,7 +562,8 @@ write_gnu_long_link (struct tar_stat_info *st, const char *p, char type)
   GNAME_TO_CHARS (tmpname, header->header.gname);
   free (tmpname);
 
-  strcpy (header->header.magic, OLDGNU_MAGIC);
+  strcpy (header->buffer + offsetof (struct posix_header, magic),
+	  OLDGNU_MAGIC);
   header->header.typeflag = type;
   finish_header (st, header, -1);
 
@@ -899,7 +900,8 @@ start_header (struct tar_stat_info *st)
     case OLDGNU_FORMAT:
     case GNU_FORMAT:   /*FIXME?*/
       /* Overwrite header->header.magic and header.version in one blow.  */
-      strcpy (header->header.magic, OLDGNU_MAGIC);
+      strcpy (header->buffer + offsetof (struct posix_header, magic),
+	      OLDGNU_MAGIC);
       break;
 
     case POSIX_FORMAT:

src/list.c

@@ -122,7 +122,7 @@ transform_stat_info (int typeflag, struct tar_stat_info *stat_info)
   if (typeflag == GNUTYPE_VOLHDR)
     /* Name transformations don't apply to volume headers. */
     return;
-  
+
   transform_member_name (&stat_info->file_name, XFORM_REGFILE);
   switch (typeflag)
     {
@@ -592,7 +592,9 @@ decode_header (union block *header, struct tar_stat_info *stat_info,
       else
 	format = USTAR_FORMAT;
     }
-  else if (strcmp (header->header.magic, OLDGNU_MAGIC) == 0)
+  else if (strcmp (header->buffer + offsetof (struct posix_header, magic),
+		   OLDGNU_MAGIC)
+	   == 0)
     format = hbits ? OLDGNU_FORMAT : GNU_FORMAT;
   else
     format = V7_FORMAT;