Backpatching overwrote already backpatched instructions, which made it
skip the hpop instruction; possibly not only a problem for handlers,
but this is one known case when it happened.


mysql-test/r/sp-error.result:
  New testcase for BUG#3287
mysql-test/t/sp-error.test:
  New testcase for BUG#3287
sql/sp_head.cc:
  Made the debug printout of hpush_jump instructions somewhat clearer.
sql/sp_head.h:
  Don't backpatch the same instruction more than once.