x86/cpu: Avoid running off the end of an AMD erratum table

In the Linux kernel, the following vulnerability has been resolved:

x86/cpu: Avoid running off the end of an AMD erratum table

The NULL array terminator at the end of erratum_1386_microcode was

removed during the switch from x86_cpu_desc to x86_cpu_id. This

causes readers to run off the end of the array.

Replace the NULL.