Fix recursive crash prints on FVP AEM model

Fix recursive crash prints on FVP AEM model

This patch fixes an issue in the cpu specific register reporting
of FVP AEM model whereby crash reporting itself triggers an exception
thus resulting in recursive crash prints. The input to the
'size_controlled_print' in the crash reporting framework should
be a NULL terminated string. As there were no cpu specific register
to be reported on FVP AEM model, the issue was caused by passing 0
instead of NULL terminated string to the above mentioned function.

Change-Id: I664427b22b89977b389175dfde84c815f02c705a