Bug #29494 Field packet with NULL fields crashes libmysqlclient.

unpack_fields() didn't expect NULL_LENGHT in the field's descriptions.
In this case we get NULL in the resulting string so cannot use
strdup_root to make a copy of it.
strdup_root changed with strmake_root as it's NULL-safe


sql-common/client.c:
  Bug #29494 Field packet with NULL fields crashes libmysqlclient
  
  strdup_root changed with strmake_root in unpack_fields()

sql-common/client.c

@@ -1176,12 +1176,12 @@ unpack_fields(MYSQL_DATA *data,MEM_ROOT *alloc,uint fields,
       /* fields count may be wrong */
       DBUG_ASSERT ((field - result) < fields);
       cli_fetch_lengths(&lengths[0], row->data, default_value ? 8 : 7);
-      field->catalog  = strdup_root(alloc,(char*) row->data[0]);
-      field->db       = strdup_root(alloc,(char*) row->data[1]);
-      field->table    = strdup_root(alloc,(char*) row->data[2]);
-      field->org_table= strdup_root(alloc,(char*) row->data[3]);
-      field->name     = strdup_root(alloc,(char*) row->data[4]);
-      field->org_name = strdup_root(alloc,(char*) row->data[5]);
+      field->catalog=   strmake_root(alloc,(char*) row->data[0], lengths[0]);
+      field->db=        strmake_root(alloc,(char*) row->data[1], lengths[1]);
+      field->table=     strmake_root(alloc,(char*) row->data[2], lengths[2]);
+      field->org_table= strmake_root(alloc,(char*) row->data[3], lengths[3]);
+      field->name=      strmake_root(alloc,(char*) row->data[4], lengths[4]);
+      field->org_name=  strmake_root(alloc,(char*) row->data[5], lengths[5]);
 
       field->catalog_length=	lengths[0];
       field->db_length=		lengths[1];
@@ -1202,7 +1202,7 @@ unpack_fields(MYSQL_DATA *data,MEM_ROOT *alloc,uint fields,
         field->flags|= NUM_FLAG;
       if (default_value && row->data[7])
       {
-        field->def=strdup_root(alloc,(char*) row->data[7]);
+        field->def=strmake_root(alloc,(char*) row->data[7], lengths[7]);
 	field->def_length= lengths[7];
       }
       else