jbd: replace potentially false assertion with if block

If an error occurs during jbd cache initialisation it is possible for the
journal_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>