flash/stm32l4x: free write_algorithm work area if no space left for the buffer

flash/stm32l4x: free write_algorithm work area if no space left for the buffer

If the remaining memory for the buffer is less than 256 bytes,
the memory allocated for the write algorithm in the target is not freed.

Fixes: ba131f30a079 (Flash driver for STM32G0xx and STM32G4xx)
Change-Id: Ic649f6c39799d76725b0c69ff3a009a3f510e17f
Signed-off-by: Tarek BOCHKATI <tarek.bouchkati@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/6486
Tested-by: jenkins
Reviewed-by: Oleksij Rempel <linux@rempel-privat.de>