Fixed bug #31019: the MOD() function and the % operator crash the server

when a divisor is less than 1 and its fractional part is very long.
For example:
1 % .123456789123456789123456789123456789123456789123456789123456789123456789123456789;

Stack buffer overflow has been fixed in the do_div_mod function.

mysql-test/r/type_decimal.result

@@ -799,3 +799,9 @@ SELECT ROUND(qty,3), dps, ROUND(qty,dps) FROM t1;
 ROUND(qty,3)	dps	ROUND(qty,dps)
 1.133	3	1.133
 DROP TABLE t1;
+SELECT 1 % .123456789123456789123456789123456789123456789123456789123456789123456789123456789 AS '%';
+%
+0.012345687012345687012345687012345687012345687012345687012345687012345687000000000
+SELECT MOD(1, .123456789123456789123456789123456789123456789123456789123456789123456789123456789) AS 'MOD()';
+MOD()
+0.012345687012345687012345687012345687012345687012345687012345687012345687000000000

mysql-test/t/type_decimal.test

@@ -408,3 +408,11 @@ INSERT INTO t1 VALUES (1.1325,3);
 SELECT ROUND(qty,3), dps, ROUND(qty,dps) FROM t1;
 
 DROP TABLE t1;
+
+#
+# Bug#31019: MOD() function and operator crashes MySQL when
+#            divisor is very long and < 1
+#
+
+SELECT 1 % .123456789123456789123456789123456789123456789123456789123456789123456789123456789 AS '%';
+SELECT MOD(1, .123456789123456789123456789123456789123456789123456789123456789123456789123456789) AS 'MOD()';

strings/decimal.c

@@ -2323,11 +2323,12 @@ static int do_div_mod(decimal_t *from1, decimal_t *from2,
     }
     if (unlikely(intg0+frac0 > to->len))
     {
-      stop1-=to->len-frac0-intg0;
+      stop1-=frac0+intg0-to->len;
       frac0=to->len-intg0;
       to->frac=frac0*DIG_PER_DEC1;
         error=E_DEC_TRUNCATED;
     }
+    DBUG_ASSERT(buf0 + (stop1 - start1) <= to->buf + to->len);
     while (start1 < stop1)
         *buf0++=*start1++;
   }