Bug#12392636 ASSERTION FAILED: SCALE >= 0 && PRECISION > 0 && SCALE <= PRECISION

Assertion happens due to missing NULL value check in
Item_func_round::fix_length_and_dec() function.
The fix: added NULL value check for second parameter.

mysql-test/r/func_math.result

@@ -540,4 +540,10 @@ ROUND(LEAST(15, -4939092, 0.2704), STDDEV('a'))
 -4939092.0000
 Warnings:
 Warning	1292	Truncated incorrect DOUBLE value: 'a'
+#
+# Bug#12392636  ASSERTION FAILED: SCALE >= 0 && PRECISION > 0 && SCALE <= PRECISION
+#
+SELECT SUM(DISTINCT (TRUNCATE((.1), NULL)));
+SUM(DISTINCT (TRUNCATE((.1), NULL)))
+NULL
 End of 5.1 tests

mysql-test/t/func_math.test

@@ -349,4 +349,10 @@ DROP TABLE t1;
 
 SELECT ROUND(LEAST(15, -4939092, 0.2704), STDDEV('a'));
 
+--echo #
+--echo # Bug#12392636  ASSERTION FAILED: SCALE >= 0 && PRECISION > 0 && SCALE <= PRECISION
+--echo #
+
+SELECT SUM(DISTINCT (TRUNCATE((.1), NULL)));
+
 --echo End of 5.1 tests

sql/item_func.cc

@@ -1972,6 +1972,9 @@ void Item_func_round::fix_length_and_dec()
   }
 
   val1= args[1]->val_int();
+  if ((null_value= args[1]->is_null()))
+    return;
+
   val1_unsigned= args[1]->unsigned_flag;
   if (val1 < 0)
     decimals_to_set= val1_unsigned ? INT_MAX : 0;