x86emul: always init mmval

... to avoid buggy read/write sizes becoming info leaks.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/arch/x86/x86_emulate/x86_emulate.c
+++ b/xen/arch/x86/x86_emulate/x86_emulate.c
@@ -2671,6 +2671,8 @@ x86_emulate(
         ea.reg = decode_register(modrm_rm, &_regs,
                                  (d & ByteOp) && !rex_prefix);
 
+    memset(mmvalp, 0xaa /* arbitrary */, sizeof(*mmvalp));
+
     /* Decode and fetch the source operand: register, memory or immediate. */
     switch ( d & SrcMask )
     {