Fix the bug of += decimal in olap engine

[ISSUE-1225] This change fix the olap engine bug of decimal agg. Using ^ instead of * to judge result is less then zero.
The result of * will be less then zero when the result is overflow. So the answer of += is incorrect.

be/src/olap/field_info.h

@@ -123,6 +123,7 @@ inline std::ostream& operator<<(std::ostream& os, const uint24_t& val) {
     return os;
 }
 
+// the sign of integer must be same as fraction
 struct decimal12_t {
     decimal12_t() : integer(0), fraction(0) {}
     decimal12_t(int64_t int_part, int32_t frac_part) {
@@ -147,7 +148,8 @@ struct decimal12_t {
             fraction += FRAC_RATIO;
         }
 
-        if (fraction * integer < 0) {
+        // if sign of fraction is different from integer
+        if ((fraction ^ integer) < 0) {
             bool sign = integer < 0;
             integer += (sign ? 1 : -1);
             fraction += (sign ? -FRAC_RATIO : FRAC_RATIO);

be/test/olap/field_info_test.cpp

@@ -0,0 +1,15 @@
+#include <gtest/gtest.h>
+#include <olap/field_info.h>
+
+TEST_F(){
+    decimal12_t a = decimal12_t(INT_MAX + 1, INT_MAX);
+    decimal12_t b = decimal12_t(0,0);
+
+    ASSERT_EQ(a, a+=b);
+    delete 
+}
+
+int main(int argc, char **argv) {
+    testing::InitGoogleTest(&argc, argv); 
+    return RUN_ALL_TESTS();
+}