cortex_a: fix a potential memory leak in cortex_a_target_create()

cortex_a: fix a potential memory leak in cortex_a_target_create()

If the function cortex_a_target_create() or the function
cortex_r4_target_create() exit for an error, the value of pointer
cortex_a get lost, causing a memory leak.

Move the allocation of cortex_a after the check on the parameters.
While there, add checks on the value returned by calloc().

Issue highlighted by clang 7.0.0.

Change-Id: Ic6eeb9c3b39922d016f6d0319eada1a97a6681f0
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/4925
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>
Tested-by: jenkins
Reviewed-by: Matthias Welwarsky <matthias@welwarsky.de>