Bug#57810 case/when/then : Assertion failed: length || !scale

ASSERT happens due to improper calculation of the max_length
in Item_func_div object, if dividend has max_length == 0 then
Item_func_div::max_length is set to 0 under some circumstances.
The fix:
If decimals == NOT_FIXED_DEC then set
Item_func_div::max_length to max possible
DOUBLE length value.

mysql-test/r/func_math.result

@@ -498,4 +498,17 @@ SELECT -9223372036854775808 MOD -1;
 SELECT -9223372036854775808999 MOD -1;
 -9223372036854775808999 MOD -1
 0
+#
+# Bug#57810 case/when/then : Assertion failed: length || !scale
+#
+SELECT CASE(('')) WHEN (CONVERT(1, CHAR(1))) THEN (('' / 1)) END;
+CASE(('')) WHEN (CONVERT(1, CHAR(1))) THEN (('' / 1)) END
+NULL
+CREATE TABLE t1 SELECT CAST((CASE(('')) WHEN (CONVERT(1, CHAR(1))) THEN (('' / 1)) END) AS CHAR) as C;
+SHOW CREATE TABLE t1;
+Table	Create Table
+t1	CREATE TABLE `t1` (
+  `C` varchar(23) DEFAULT NULL
+) ENGINE=MyISAM DEFAULT CHARSET=latin1
+DROP TABLE t1;
 End of 5.1 tests

mysql-test/t/func_math.test

@@ -315,4 +315,13 @@ SELECT -9223372036854775808 DIV -1;
 SELECT -9223372036854775808 MOD -1;
 SELECT -9223372036854775808999 MOD -1;
 
+--echo #
+--echo # Bug#57810 case/when/then : Assertion failed: length || !scale
+--echo #
+
+SELECT CASE(('')) WHEN (CONVERT(1, CHAR(1))) THEN (('' / 1)) END;
+CREATE TABLE t1 SELECT CAST((CASE(('')) WHEN (CONVERT(1, CHAR(1))) THEN (('' / 1)) END) AS CHAR) as C;
+SHOW CREATE TABLE t1;
+DROP TABLE t1;
+
 --echo End of 5.1 tests

sql/item_func.cc

@@ -1329,9 +1329,14 @@ void Item_func_div::fix_length_and_dec()
   {
     decimals=max(args[0]->decimals,args[1]->decimals)+prec_increment;
     set_if_smaller(decimals, NOT_FIXED_DEC);
-    max_length=args[0]->max_length - args[0]->decimals + decimals;
     uint tmp=float_length(decimals);
-    set_if_smaller(max_length,tmp);
+    if (decimals == NOT_FIXED_DEC)
+      max_length= tmp;
+    else
+    {
+      max_length=args[0]->max_length - args[0]->decimals + decimals;
+      set_if_smaller(max_length,tmp);
+    }
     break;
   }
   case INT_RESULT: