Fixing "mtr func_math" failure in the test for MDEV-17643

Adding an intermediate volatile variable to avoid using co-processor registers
on some platforms (e.g. 32-bit x86).
This change makes test results stable accross all platforms.

sql/item_sum.cc

@@ -1824,8 +1824,9 @@ static void variance_fp_recurrence_next(double *m, double *s, ulonglong *count,
   else
   {
     double m_kminusone= *m;
-    *m= m_kminusone + (nr - m_kminusone) / (double) *count;
-    *s= *s + (nr - m_kminusone) * (nr - *m);
+    volatile double diff= nr - m_kminusone;
+    *m= m_kminusone + diff / (double) *count;
+    *s= *s + diff * (nr - *m);
   }
 }