cortex_m: dwt_num_comp should be set to zero in cortex_m_dwt_free()

cortex_m: dwt_num_comp should be set to zero in cortex_m_dwt_free()

A segmentation fault in cortex_m_endreset_event() is sometimes raised
with very broken target like Kinetis Kx with erased flash and active WDOG.
Debugging revealed that cortex_m->dwt_num_comp is 4 and
dwt_list is NULL at cortex_m:290

Change-Id: I229c59d6da13d816df513d1dbb19968e4b5951e2
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-on: http://openocd.zylin.com/2989
Reviewed-by: Thomas Schmid <thomas@rfranging.com>
Tested-by: jenkins
Reviewed-by: Paul Fertser <fercerpav@gmail.com>