(And some minor correction of cursor open)


sql/sp_head.cc:
  Detect some errors that doesn't result in a non-zero return code in
  the SP execution loop.
  (Also corrected the cursor post_open() call.)
sql/sp_rcontext.cc:
  Corrected the semantics of cursor post_open().
sql/sp_rcontext.h:
  Corrected the semantics of cursor post_open().