Diagnostics_area::set_ok_status at PREPARE

The problem occured during processing of stored routines. 
Routines are loaded from mysql.proc, parsed and put into the sp cache by
sp_cache_routine().  The assert occured because the return value from
sp_cache_routine() was not checked for top level CALLs. This meant that any
errors during sp_cache_routine() went unoticed and triggered the assert when
my_ok() was later called.

This is a regression introduced by the patch for Bug#30977, only visible in
source trees with MDL and using debug builds of the server.

This patch fixes the problem by checking the return value from sp_cache_routine() 
for top level CALLs and propagating any errors similar to what is done for other 
calls to sp_cache_routine().

No test case added.