Analysis
--------

'mysqld' crashes when:
a) The primary key for the system table 'proc' is dropped and
b) A stored procedure is invoked.

Ideally it is expected that system tables are not tampered.
When we access these tables for performing some operation,
we do not check if a valid key information is available.
Since the primary key is dropped, the mysqld crashes while trying
to access the key information. This behavior is observed with
other system tables(user, columns_priv, event and plugin)
as well when it's primary key is dropped. This issue is addressed
with this patch.

Fix:
---
Make sure that key information is available when the operations
accessing the system tables are invoked. In such a case, we now
report an error instead of a crash.

Note: We do not validate complete key information as it could
impact performance. Once MySQL roles is introduced, such checks
would become unnecessary.