jtag/tcl: fix incorrect memcpy in jim_newtap_expected_id

jtag/tcl: fix incorrect memcpy in jim_newtap_expected_id

Found by clang static checker.

On the very first call of jim_newtap_expected_id() pTap->expected_ids
and expected_len are null, and there's nothing to copy. This patch
changes this cryptic code to use realloc() instead.

Change-Id: Ic0b5140d08257a906f15b55a2ae64db7bc06d5f1
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-on: http://openocd.zylin.com/2562
Reviewed-by: Stian Skjelstad <stian@nixia.no>
Tested-by: jenkins