Fix handling of NULL values in decimal fields in FORMAT(). (Bug #13361)

mysql-test/r/func_str.result:
  Add new results
mysql-test/t/func_str.test:
  Add new regression test
sql/item_strfunc.cc:
  Handle NULL decimal fields in FORMAT().

mysql-test/r/func_str.result

@@ -1011,3 +1011,9 @@ t
 1000000
 1      
 drop table t1;
+create table t1 (d decimal default null);
+insert into t1 values (null);
+select format(d, 2) from t1;
+format(d, 2)
+NULL
+drop table t1;

mysql-test/t/func_str.test

@@ -665,3 +665,13 @@ select rpad(i, 7, ' ') as t from t1;
 drop table t1;
 
 # End of 4.1 tests
+
+#
+# Bug #13361: SELECT FORMAT(<decimal field with null>, 2) crashes
+#
+create table t1 (d decimal default null);
+insert into t1 values (null);
+select format(d, 2) from t1;
+drop table t1;
+
+# End of 5.0 tests

sql/item_strfunc.cc

@@ -1733,6 +1733,8 @@ String *Item_func_format::val_str(String *str)
   {
     my_decimal dec_val, rnd_dec, *res;
     res= args[0]->val_decimal(&dec_val);
+    if ((null_value=args[0]->null_value))
+      return 0; /* purecov: inspected */
     my_decimal_round(E_DEC_FATAL_ERROR, res, decimals, false, &rnd_dec);
     my_decimal2string(E_DEC_FATAL_ERROR, &rnd_dec, 0, 0, 0, str);
     str_length= str->length();