flash/nor/cfi: fix uninitialized write-mem pointer

flash/nor/cfi: fix uninitialized write-mem pointer

In flash/nor/cfi.c:835 struct cfi_info is allocated by malloc(). As
write-mem was uninitialized the pointer pointed to an out of range
address, which led to a segmentation fault and crashed openocd.
This happened during flash-command of an external flash-bank, using
cfi.
Use calloc() instead.
While on it check for NULL return and remove unnecessary initialzation.

Change-Id: I0e2ffb90559afe7f090837023428dcc06b2e29f6
Signed-off-by: Mischa Studer <mischa.studer@csa.ch>
Reviewed-on: http://openocd.zylin.com/6070
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>