flash/nor/core: fix Segmentation fault during flash write of bad formed img

flash_write_unlock() sorts sections by base address but does not check
if they overlap. In case of overlapped sections an item of padding[]
array can get negative and padding loop writes out of allocated buffer.

How to replicate: cat two copies of an ihex file to one file and try
to flash it.

Check for overlapped sections and abort write in such case.

Change-Id: I43eee7dc290a8d18faa59567b2118b88ad4bedca
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-on: http://openocd.zylin.com/4397
Tested-by: jenkins
Reviewed-by: Andreas Bolsch <hyphen0break@gmail.com>