flash/nor/lpc2000: free allocated working area when target_write fails

flash/nor/lpc2000: free allocated working area when target_write fails

In some circumstances (e.g. inappropriate jtag clock)
target_write_memory in lpc2000_iap_working_area_init might fail. The
allocated working area should be freed inside
lpc2000_iap_working_area_init in this error case.

This was leading to a weird segfault due to stack corruption later
when reset was executed.

Reported by quitte (Jonas Meyer).

Change-Id: Ia2ed42a9970a4d771727fd516a6eea88e9b859e2
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-on: http://openocd.zylin.com/2696
Tested-by: jenkins