target,flash: allow target_free_working_area on NULL area pointer

target,flash: allow target_free_working_area on NULL area pointer

Standard C library free() allows NULL pointer as a parameter.

Change target_free_working_area() to conform this convention.

Remove NULL pointer tests before target_free_working_area() calls.

While on it add missing setting pointer to NULL after target_free_working_area().

Change-Id: I7c692ab04a9933398ba5bc614723ad0bdecb87b3
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-on: https://review.openocd.org/c/openocd/+/6712
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>