flash/nor/tcl: fix segfault on write_image misuse

"flash write_image (unlock|erase)" (without filename) was causing
segmentation fault in strcmp(). This patch fixes it.

Change-Id: Ia3b8c796f4107621d78605c02d2b8e123b1b207b
Reported-by: ajkroll][ <ajkroll][@irc.freenode.net>
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-on: http://openocd.zylin.com/1970


Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>