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/r/func_math.result

@@ -408,3 +408,22 @@ a DIV 2
 0
 DROP TABLE t1;
 End of 5.0 tests
+SELECT 1e308 + 1e308;
+1e308 + 1e308
+NULL
+SELECT -1e308 - 1e308;
+-1e308 - 1e308
+NULL
+SELECT 1e300 * 1e300;
+1e300 * 1e300
+NULL
+SELECT 1e300 / 1e-300;
+1e300 / 1e-300
+NULL
+SELECT EXP(750);
+EXP(750)
+NULL
+SELECT POW(10, 309);
+POW(10, 309)
+NULL
+End of 5.1 tests