jtag/tcl: fix memory leak in command 'irscan'

jtag/tcl: fix memory leak in command 'irscan'

If the function parse_u64() fails, we jump to return, thus leaking
the memory just allocated in 'v'.
Issue identified by clang.

Move earlier the call to parse_u64() and the associated test,
before memory allocation.

While there, fix a possible NULL pointer dereferencing in case the
calloc() fails, by testing for allocation failure.

Change-Id: I6a77ee17aceb282bbdfefe7cdafeba2e0e7012f1
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/5692
Tested-by: jenkins
Reviewed-by: Tarek BOCHKATI <tarek.bouchkati@gmail.com>