Fix for bug #18643: crazy UNCOMPRESS().

mysql-test/r/func_compress.result:
  Fix for bug #18643: crazy UNCOMPRESS().
    - test-case
mysql-test/t/func_compress.test:
  Fix for bug #18643: crazy UNCOMPRESS().
    - test-case
sql/item_strfunc.cc:
  Fix for bug #18643: crazy UNCOMPRESS().
    - set null_value.

mysql-test/r/func_compress.result

@@ -72,3 +72,10 @@ set @@max_allowed_packet=1048576*100;
 select compress(repeat('aaaaaaaaaa', IF(XXX, 10, 10000000))) is null;
 compress(repeat('aaaaaaaaaa', IF(XXX, 10, 10000000))) is null
 0
+create table t1(a blob);
+insert into t1 values(NULL), (compress('a'));
+select uncompress(a), uncompressed_length(a) from t1;
+uncompress(a)	uncompressed_length(a)
+NULL	NULL
+a	1
+drop table t1;

mysql-test/t/func_compress.test

@@ -45,4 +45,13 @@ set @@max_allowed_packet=1048576*100;
 --replace_result "''" XXX "'1'" XXX
 eval select compress(repeat('aaaaaaaaaa', IF('$LOW_MEMORY', 10, 10000000))) is null;
 
+#
+# Bug #18643: problem with null values
+#
+
+create table t1(a blob);
+insert into t1 values(NULL), (compress('a'));
+select uncompress(a), uncompressed_length(a) from t1;
+drop table t1;
+
 # End of 4.1 tests

sql/item_strfunc.cc

@@ -2850,6 +2850,7 @@ String *Item_func_uncompress::val_str(String *str)
 
   if (!res)
     goto err;
+  null_value= 0;
   if (res->is_empty())
     return res;