Add a test if the pointer to the target_init function in the target struct
is set before the function pointer is used.

Change-Id: Ie4ea542f64f35efce8c5bce2ced9b881bf283ec1
Signed-off-by: Mathias K <kesmtp@freenet.de>
Reviewed-on: http://openocd.zylin.com/241


Tested-by: jenkins
Reviewed-by: Øyvind Harboe <oyvindharboe@gmail.com>