jbd2: replace potentially false assertion with if block

If an error occurs during jbd2 cache initialisation it is possible for the
journal_head_cache to be NULL when jbd2_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 jbd2 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: Mingming Cao <cmm@us.ibm.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>