x86emul: always fill x86_insn_modrm()'s outputs

The function is rather unlikely to be called for insns which don't have
ModRM bytes, and hence addressing Coverity's recurring complaint of
callers potentially consuming uninitialized data when they know that
certain opcodes have ModRM bytes can be suppressed this way without
unduly adding overhead to fast paths.

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
@@ -8017,8 +8017,14 @@ x86_insn_modrm(const struct x86_emulate_
 {
     check_state(state);
 
-    if ( state->modrm_mod > 3 )
+    if ( unlikely(state->modrm_mod > 3) )
+    {
+        if ( rm )
+            *rm = ~0U;
+        if ( reg )
+            *reg = ~0U;
         return -EINVAL;
+    }
 
     if ( rm )
         *rm = state->modrm_rm;