MDEV-4005 fix.

Field matching fixed.
DBUG_ASSERT fixed.

mysql-test/r/cassandra.result

@@ -580,3 +580,14 @@ select rowkey, column_json(dyn) from t1;
 rowkey	column_json(dyn)
 delete from t1;
 DROP TABLE t1;
+#
+# MDEV-4005 #Server crashes on creating a Cassandra table
+# with a mix of static and dynamic columns
+#
+DROP TABLE IF EXISTS t1, t2;
+CREATE TABLE t1 (
+pk int primary key,
+col_int int,
+dyncol blob DYNAMIC_COLUMN_STORAGE=yes
+) ENGINE=cassandra keyspace='bug' thrift_host = '127.0.0.1' column_family='cf1';
+drop table t1;

mysql-test/t/cassandra.test

@@ -664,6 +664,42 @@ select rowkey, column_json(dyn) from t1;
 delete from t1;
 DROP TABLE t1;
 
+--echo #
+--echo # MDEV-4005 #Server crashes on creating a Cassandra table
+--echo # with a mix of static and dynamic columns
+--echo #
+--disable_warnings
+DROP TABLE IF EXISTS t1, t2;
+--enable_warnings
+
+--remove_files_wildcard $MYSQLTEST_VARDIR cassandra_test_cleanup.cql
+--write_file $MYSQLTEST_VARDIR/cassandra_test_cleanup.cql
+drop keyspace bug;
+EOF
+--error 0,1,2
+--system cqlsh -3 -f $MYSQLTEST_VARDIR/cassandra_test_cleanup.cql
+
+--remove_files_wildcard $MYSQLTEST_VARDIR cassandra_test_init.cql
+--write_file $MYSQLTEST_VARDIR/cassandra_test_init.cql
+
+CREATE KEYSPACE bug
+  WITH strategy_class = 'org.apache.cassandra.locator.SimpleStrategy'
+  AND strategy_options:replication_factor='1';
+
+USE bug;
+create columnfamily cf1 ( pk int primary key, col_int int, a bigint );
+EOF
+
+--system cqlsh -3 -f $MYSQLTEST_VARDIR/cassandra_test_init.cql
+
+
+CREATE TABLE t1 (
+ pk int primary key,
+ col_int int,
+ dyncol blob DYNAMIC_COLUMN_STORAGE=yes
+) ENGINE=cassandra keyspace='bug' thrift_host = '127.0.0.1' column_family='cf1';
+
+drop table t1;
 
 ############################################################################
 ## Cassandra cleanup

storage/cassandra/ha_cassandra.cc

@@ -1517,6 +1517,7 @@ bool ha_cassandra::setup_field_converters(Field **field_arg, uint n_fields)
           DBUG_RETURN(true);
         }
         (*conv)->field= *field;
+        break;
       }
     }
     if (dyncol_set && !(*field)) // is needed and not found
@@ -1611,7 +1612,7 @@ void ha_cassandra::free_field_converters()
     for (uint i=0; i < n_field_converters; i++)
       if (field_converters[i])
       {
-        DBUG_ASSERT(!dyncol_set || i == dyncol_field);
+        DBUG_ASSERT(!dyncol_set || i != dyncol_field);
         delete field_converters[i];
       }
     my_free(field_converters);