Bug #11792200 - DIVIDING LARGE NUMBERS CAUSES STACK CORRUPTIONS

This was a buffer overrun in do_div_mod(), overwriting the internal buffer
of auto variable 'tmp' in Item_func_int_div::val_int.
Result on windows: 'this' is set to zero, and crash.
Ran fine on other platforms (no valgrind warnings),
but this is undefined behaviour on any platform of course.

include/decimal.h:
  Add const qualifiers to function prototypes which are used by sql/my_decimal.h
mysql-test/r/func_math.result:
  New test case.
mysql-test/t/func_math.test:
  New test case.
sql/my_decimal.h:
  Remove several C-style casts:
   - some of the were up-casts, and thus un-necessary
   - some of them should have been const-casts, but it is better to make the
     underlying library functions in (decimal.[h|c]) const instead.
strings/decimal.c:
  Check for buffer overrun in do_div_mod()
  Add const qualifiers to functions which are used by sql/my_decimal.h

mysql-test/r/func_math.result

@@ -656,3 +656,11 @@ Warning	1366	Incorrect decimal value: '' for column '' at row -1
 SELECT 1 div null;
 1 div null
 NULL
+#
+# Bug #11792200 - DIVIDING LARGE NUMBERS CAUSES STACK CORRUPTIONS
+#
+select (1.175494351E-37 div 1.7976931348623157E+308);
+(1.175494351E-37 div 1.7976931348623157E+308)
+0
+Warnings:
+Warning	1292	Truncated incorrect DECIMAL value: ''

mysql-test/t/func_math.test

@@ -500,3 +500,8 @@ SELECT ((@a:=@b:=1.0) div (@b:=@a:=get_format(datetime, 'usa')));
 --echo # Bug #59498 div function broken in mysql-trunk
 --echo #
 SELECT 1 div null;
+
+--echo #
+--echo # Bug #11792200 - DIVIDING LARGE NUMBERS CAUSES STACK CORRUPTIONS
+--echo #
+select (1.175494351E-37 div 1.7976931348623157E+308);