Fix for bug #27041477 "ASSERTION `HAS_PRELOCKING_LIST || THD->MDL_CONTEXT.OWNS_EQUAL_OR_STRONGER_LOCK".

Under LOCK TABLES attempt to execute DML statement on a table with
foreign keys led to assertion failure if this statement was
incompatible with the mode in which tables were locked.

The problem was that assertion in part of prelocking algorithm
responsible for handling tables involved in foreign keys under
LOCK TABLES did not take into account several scenarios in which
these tables might have been not locked properly.

This patch solves the problem by replacing problematic assertion
with check if appropriate metadata locks are acquired. If necessary
locks are missing we now fail with an error instead failing on assert.