x86/ucode/amd: Fix OoB read in cpu_request_microcode()

x86/ucode/amd: Fix OoB read in cpu_request_microcode()

verify_patch_size() is a maximum size check, and doesn't have a minimum bound.

If the microcode container encodes a blob with a length less than 64 bytes,
the subsequent calls to microcode_fits()/compare_header() may read off the end
of the buffer.

Fixes: 4de936a38a ("x86/ucode/amd: Rework parsing logic in cpu_request_microcode()")
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>