stm32lx: don't allow part_info to be uninitialized

It's possible for us to fail to read the part ID code so make sure that
part_info is initialized to NULL before attempting to do so, otherwise
we could proceed and use it uninitialized and then segfault.

Change-Id: I0a3f3d3947690b66f0981b5046340449521e0b33
Signed-off-by: Jack Peel <jack.peel@synapse.com>
Signed-off-by: Andrey Yurovsky <yurovsky@gmail.com>
Reviewed-on: http://openocd.zylin.com/2276
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>