target/i386: check for empty register in FXAM

The fxam instruction returns the wrong result after fdecstp or after
an underflow.  Check fptags to handle this.

Reported-by: <chengang@emindsoft.com.cn>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

target/i386/fpu_helper.c

@@ -991,7 +991,11 @@ void helper_fxam_ST0(CPUX86State *env)
         env->fpus |= 0x200; /* C1 <-- 1 */
     }
 
-    /* XXX: test fptags too */
+    if (env->fptags[env->fpstt]) {
+        env->fpus |= 0x4100; /* Empty */
+        return;
+    }
+
     expdif = EXPD(temp);
     if (expdif == MAXEXPD) {
         if (MANTD(temp) == 0x8000000000000000ULL) {