ROBUST AGAINST BUGS IN CALLERS".

Both MDL subsystems and Table Definition Cache code assume
that callers ensure that names of objects passed to them are
not longer than NAME_LEN bytes. Unfortunately due to bugs in
callers this assumption might be broken in some cases. As
result we get nasty bugs causing buffer overruns when we
construct MDL key or TDC key from object names.

This patch makes MDL and TDC code more robust against such
bugs by ensuring that we always checking size of result
buffer when constructing MDL and TDC keys. This doesn't
free its callers from ensuring that both db and table names
are shorter than NAME_LEN bytes. But at least these steps
prevents buffer overruns in case of bug in caller, replacing
them with less harmful behavior.

This is 5.6+ version of patch.

Changed code of MDL_key::mdl_key_init() to take into account
size of buffer for the key. Did the same thing for
create_table_def_key() function. Added asserts ensuring that
callers won't pass arguments longer than NAME_LEN to them.

Adjusted code constructing TDC keys directly to either use
create_table_def_key() or get_table_def_key(). To support
this change changed assert in get_table_def_key() in such
way that it can be for table list elements which were used
for opening of a view.