al_head_cache to be NULL when journal_destroy_journal_head_cache is
called.  Replace the J_ASSERT with an if block to handle the situation
correctly.

Note that even with this fix things will break badly if jbd is statically
compiled in and cache initialisation fails.

Signed-off-by: Duane Griffin <duaneg@dghda.com
Cc: <linux-ext4@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
