Fix for bug #31236: Inconsistent division by zero behavior for

floating point numbers

Some math functions did not check if the result is a valid number
(i.e. neither of +-inf or nan).

Fixed by validating the result where necessary and returning NULL in
case of invalid result.

mysql-test/t/strict.test

@@ -822,7 +822,6 @@ INSERT INTO t1 (col2) VALUES (-1.1E-3);
 INSERT INTO t1 (col1) VALUES ('+1.8E+309');
 --error 1264
 INSERT INTO t1 (col2) VALUES ('-1.2E-3');
---error 1264
 UPDATE t1 SET col1 =col1 * 5000 WHERE col1 > 0;
 --error 1365
 UPDATE t1 SET col2 =col2 / 0 WHERE col2 > 0;