x86/microcode/AMD: Check the equivalence table size when scanning it

Currently, the code scanning the CPU equivalence table read from a
microcode container file assumes that it actually contains a terminating
zero entry.

Check also the size of this table to make sure that no reads past its
end happen, in case there's no terminating zero entry at the end of the
table.

 [ bp: Adjust to new changes. ]

Signed-off-by: Maciej S. Szmigiero <mail@maciej.szmigiero.name>
Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: x86@kernel.org
Link: https://lkml.kernel.org/r/20181107170218.7596-16-bp@alien8.de