target, arm_adi_v5: catch two allocation errors

target, arm_adi_v5: catch two allocation errors

Command
	mdw 0 0x40000000
triggers Segmentation fault on an arm.
Size parameter is a nonsence that may happen e.g. if you
mistype mdw instead of mww.

Add checking for calloc() NULL return in mdb/h/w.

Use calloc() instead of malloc() as multiplication
count * sizeof(uint32_t) overflows for size >= 0x40000000.

Change-Id: I968c944d863d1173ef932a7077d526fccb9381ae
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-on: http://openocd.zylin.com/4349
Tested-by: jenkins
Reviewed-by: Matthias Welwarsky <matthias@welwarsky.de>