The cause of the crash is an assertion failure that we do not emit 
an error message (grant not found) and then return "ok".  The 
assertion is valid, and we were ignoring the buggy behavior prior 
to the "Diagnostics" result-verification.

Use an error handler to mutate innocuous missing-grant errors, when 
removing routines, into warnings.


mysql-test/r/drop.result:
  Show that the crash disappears.  Also prepare for the larger bug to
  be fixed with only minor changes to this test.
mysql-test/t/drop.test:
  Show that the crash disappears.  Also prepare for the larger bug to
  be fixed with only minor changes to this test.
sql/sql_acl.cc:
  Disable a segment of code that makes a faulty assumption
  about the existence of a routine's defining user, until that 
  assumption becomes true.
  
  Push a new handler onto the error-handler stack, so that when 
  removing a routine, a missing ACL grant is now a warning
  instead of an error.  If any unexpected error is raised then tell
  the caller.