Issue:
------
Prefix for externally stored columns were being stored in online_log when a
table is altered and alter causes table to be rebuilt. Space in online_log is
limited and if length of prefix of externally stored columns is very big, then
it is being written to online log without making sure if it fits. This leads to
memory corruption.

Fix:
----
After fix for Bug#16544143, there is no need to store prefixes of externally
stored columnd in online_log. Thus remove the code which stores column prefixes
for externally stored columns. Also, before writing anything on online_log,
make sure it fits to available memory to avoid memory corruption.

Read RB page for more details.
Reviewed-by: Annamalai Gurusami <annamalai.gurusami@oracle.com>
RB: 18239